2 in an amount of 70 mass% or more, and an alkali metal oxide and an alkaline earth metal oxide in a total amount of 1 to 10 mass%, with the remainder (after excluding the SiC>2, the alkali metal oxide and the alkaline earth metal oxide) being a neutral oxide and/or an acidic oxide other than SiC>2. Specifically, the remainder is most preferably an aluminosilicate-based material comprising A1203. [0075] The above composition, particularly, with the remainder being an aluminosilicate-based material comprising AI2O3, has a softening point of 1000 to 1400°C (in the present invention, the term "softening" means a state in which a deformation (different from breaking) occurs in an outer shape under a pressure of 2.5 MPa or less), and facilitates softening/deformation of the intermediate layer in a high-temperature region to increase a compressable amount under a hot condition. [0076] The hollow refractory aggregate exhibits compressibility based on brittle failure by a pressure of 2.5 MPa or less, in a low-temperature region of less than about 1000°C before softening. Further, the hollow refractory aggregate includes the vitreous composition containing an alkali metal oxide and an alkaline earth metal oxide in a total amount of 1 to 10 mass%. This facilitates softening/deformation of the hollow refractory aggregate in a high-temperature range of about 1000 to 1500°C (temperature of molten steel) to allow the hollow refractory aggregate to be reduced in volume so as to contribute to realization of a stress absorption capability and a hot strength. [0077] If the amount of SiCh is less than 70 mass% and the total amount of an alkali metal oxide and an alkaline earth metal oxide is greater than 10 mass%, or if the amount of SiC>2 is 70 mass% or more and the total amount of an alkali metal oxide and an alkaline earth metal oxide is greater than 10 mass%, viscosity of molten glass is likely to cause a problem in preparing a hollow raw material, and low high-temperature viscosity is likely to cause a problem in a bonding force for holding the inner bore-side layer. Further, if the amount of Si02 is less than 70 mass% and the total amount of an alkali metal oxide and an alkaline earth metal oxide is less than 1 mass%, or if the amount of SiCh is 70 mass% or more and the total amount of an alkali metal oxide and an alkaline earth metal oxide is less than 1 mass%, the vitreous composition tends to have an excessively high viscosity to cause a problem in preparing a hollow raw material and a problem of deterioration in softening/deformation behavior and in adhesive force for holding the inner bore-side layer, in a high-temperature region. [0078] In specifying a composition of the hollow refractory aggregate in the present invention, a volatile material and a combustible material in a non-oxidation atmosphere are not included therein. Specifically, the composition is specified based on a sample after a heat treatment in a non-oxidation atmosphere at about 600°C or more. [0079] Before the hollow refractory aggregate is reduced in volume through softening and breaking by stress, it exists in the refractory composition as an aggregate having a volume. Thus, the hollow refractory aggregate allows the intermediate layer to exhibit and maintain a higher structural strength and a higher stress distribution capability, while significantly reducing intrusion or penetration of extraneous fluid, such as molten metal or air, as compared, for example, with a conventional mortar initially having a void therein. Therefore, the hollow refractory aggregate can also contribute to stability of the intermediate layer itself, stability of a layer structure of the continuous casting nozzle, etc., as described later. [0080] The intermediate layer is required to have an ability to prevent displacement, peel-off, breaking, etc., of the inner bore-side layer, even if the inner bore-side layer receives an external force in various stages of transport, installation, preheating, and passing of molten steel of the continuous casting nozzle. [0081] In a mortar having a large volume of void simply formed in a refractory matrix structure, the structure is broken after shrinkage to cause embrittlement of the intermediate layer itself and deterioration in bonding strength, resulting in breakup of the intermediate layer itself. Consequently, it leads to a significant high risk of causing peel-off and breaking of the inner bore-side layer, intrusion of molten steel, etc., between the inner bore-side and outer periphery-side layers, etc. [0082] It has been found that troubles related to the inner bore-side layer during casting operation are mostly caused by insufficiency in bonding capability of the intermediate layer. Thus, in case where the intermediate layer is required to have a high bonding capability, it is necessary to allow the inner bore-side layer to be stably bonded to the outer periphery-side layer during passing of molten steel at high temperatures, while maintain a given structural strength through the intermediate layer, particularly, after the intermediate layer is compressed by thermal expansion of the inner bore-side layer. [0083] As mentioned above, the compressibility of the refractory material of the intermediate layer of the present invention is primarily achieved by breaking/deformation of the hollow refractory aggregate, and therefore a matrix structure has a higher strength and a higher density as compared with the conventional mortar. Thus, deterioration in structural embrittlement (deterioration in breaking strength) and deterioration in bonding strength are significantly suppressed. [0084] Further, when a given stress is applied to the refractory structure of the intermediate layer, only a part of the hollow refractory aggregate necessary for compressibility is broken, or the hollow refractory aggregate is softened and deformed under a hot condition in a non-oxidation atmosphere at a temperature of 1000 to 1500°C (temperature of molten steel), to relax the stress to prevent breaking or the like of the continuous casting nozzle. Simultaneously, the remaining part of the hollow refractory aggregate unnecessary for compressibility maintains a shape thereof to serve as a backbone of the refractory material of the intermediate layer. [0085] During breaking or softening/deformation of the hollow refractory aggregate, only a part of the shells of the hollow refractory aggregate particles receiving a compression stress, for example, from a matrix portion therearound, are reduced in volume in such a manner that shell walls of the hollow refractory aggregate particles are broken or deformed toward respective insides of the hollow refractory aggregate particles. The hollow refractory aggregate particles are dispersed in the matrix structure. Thus, the breaking or softening/deformation of the hollow refractory aggregate never causes a large local deformation in the matrix structure or breaking of the matrix structure at a level precluding a possibility to maintain shape retainability which would otherwise occur in the conventional high-porosity mortal. [0086] Accordingly, the hollow refractory aggregate can exist while being in close contact with a portion of the matrix structure therearound, i.e., without forming a void in the portion of the matrix structure therearound, and maintaining a configuration as an aggregate in an unbroken refractory structure. Thus, the intermediate layer can maintain a strong and dense structure while almost preventing a formation of pores and voids in a contact surface between the inner bore-side and outer periphery-side layers, and stably maintain bond between the inner bore-side and outer periphery-side layers while receiving an external force by expansion of the inner bore-side layer. [0087] However, it is desirable to more positively impart a bonding capability to a contact surface between the intermediate layer and each of the inner bore-side and outer periphery-side layers. [0088] Therefore, in the present invention, a formation of a product, such as carbide, through a reaction of a metal at high temperatures, is used as a means to enhance the bonding capability of the intermediate layer. Specifically, the refractory material for the intermediate layer of the present invention contains, as a percentage with respect to a total amount of the remainder after excluding the hollow refractory aggregate contained in an amount of 10 to 75 volume%, one or more (hereinafter referred to collectively as "specific metal") selected from the group consisting of Al, Si, Mg and an alloy comprising any combination of two or more thereof, in a total amount of 0.5 to 15 mass% in terms of only a content of the metals, and carbon in an amount of 2 to 99.5 mass%. In specifying a composition of the hollow refractory aggregate in the present invention, a volatile material and a combustible material in a non-oxidation atmosphere are not included therein. Specifically, the composition is specified based on a sample after a heat treatment in a non-oxidation atmosphere at a temperature of about 600 to 800°C. [0089] As above, the specific metal and carbon coexist in the remainder in a dispersed manner. This makes it possible to enhance a bonding strength of the intermediate layer and a binding strength of the refractory structure itself under a hot condition at a temperature of about 800°C or more, particularly about 1000°C or more, in cooperation with carbon bond derived from a resin or the like generally for use in binding between components of a refractory material and ensuring shape retainability. [0090] This function is considered as follows. Through coexistence with carbon, the specific metal is exposed to a reduction atmosphere during casting operation. Thus, the specific metal is vaporized as Mg-based gas and/or Al-based gas, and a part of the vaporized gas is deposited and bonded (hereinafter referred to simply as "deposited") to pores or other portion in the refractory structure where a partial pressure of oxygen would be relatively high, in the form of metal carbide and/or metal oxide. The oxide of the specific metal is also deposited concentratedly on a portion other than the pores formed inside the refractory material, such as a portion adjacent to the refractory material, particularly, pores and voids adjacent to a contact interface between the intermediate layer and molten steel containing oxygen components. [0091] A structural strength and bonding capability of the refractory material of the intermediate layer at a temperature less than about 800°C are primarily provided by the carbon bond derived from resin or the like. At a high temperature of about 800°C or more, particularly about 1000°C or more, a binding structure based on a carbide created by a reaction between the specific metal and carbon, an oxide created by the deposit, etc., is added to the carbon bond derived from resin or the like, etc., to enhance bindability. [0092] In the above manner, an internal strength of the refractory structure of the intermediate layer is enhanced, and a bonding force between the inner bore-side layer and the outer periphery-side layer is enhanced. In addition, a significant effect of preventing intrusion/penetration of molten steel and other foreign substances into the intermediate layer is obtained (the binding structure based on the deposit will hereinafter be also referred to as "re-binding structure"). [0093] In the refractory material of the present invention, even if the hollow refractory aggregate is broken or deformed and reduced in volume, a portion of a matrix structure other than the hollow refractory aggregate is not largely damaged. Further, even if damage occurs in a part of the binding structure and the matrix structure, the re-binding structure is formed to contribute to reproducing or reinforcing the binding structure for the matrix of the intermediate layer, and enhancing the bonding strength between the inner bore-side and outer periphery-side layers. Consequently, the bonding strength is enhanced without deterioration at a high temperature of about 1000°C or more. [0094] In the present invention, the above advantageous effects of the hollow refractory aggregate, the specific metal and the carbon are absolutely different from those of the conventional techniques, such as a mortal-based technique, where a large volume or size of void exists from a time before initiation of receiving of molten steel, and only binding derived from resin or the like is exhibited in an initial stage of passing of molten steel, whereafter structural compression and breaking are promoted. [0095] The bonding capability can be quantitatively expressed by bonding strength, as follows. Preferably, with respect to each of the inner bore-side layer and the outer periphery-side layer, the intermediate layer has a bonding strength of 0.01 to 1.5 MPa, as measured under a hot condition in a non-oxidation atmosphere at a temperature of 1000 to 1500°C (temperature of molten steel). As a prerequisite to having the bonding strength, it is understood that the intermediate layer itself has a structural strength equal to or greater than the bonding strength. Thus, the following description will be made about only the bonding strength. [0096] If the bonding strength is less than 0.01 MPa, an ability to holding the inner bore-side layer becomes lower, which is likely to cause peel-off of the inner bore-side layer, due to shock at start of passing of molten steel or a change in flow rate of molten steel, or when a local melting loss occurs in the inner bore-side layer. If the bonding strength is greater than 1.5 MPa, a strength of an internal structure of the intermediate layer is also increased to the same level as that of the bonding strength to spoil compressibility of the intermediate layer. Thus, a thermal expansion force of the inner bore-side layer is likely to be transmitted to the outer periphery-side layer without being relaxed, and particularly likely to cause splitting or cracking of the outer periphery-side layer. [0097] The bonding strength can be evaluated as a compression shear strength S. Specifically, as shown in FIG. 2, a tubular sample having a three-layer structure in which an inner bore-side layer 2 is provided inside an outer periphery-side layer 3(4) through an intermediate layer 1 is placed on a table 8, and evenly heated under a given hot condition for a given holding time. Then, a maximum load P (N) and a displacement of the inner bore-side layer are measured while pressing only a top surface of the inner bore-side layer by a crosshead 9 moved at a speed of 0.001 to 0.1 mm/sec, and the compression shear strength S is calculated according to the following Formula 4: S (Pa) = P / A, wherein A is a bonded area (m2) of the inner bore-side layer to the intermediate layer. [0098] A shape of the sample is not particularly limited as long as it is a tubular shape. The sample may be cut out from an actual nozzle, and subjected to the measurement. If the bonded area A is increased, the maximum load P is also increased. Thus, a maximum height dimension of the sample is preferably set to 100 mm. The measurement is performed at a minimum temperature of 1000°C and in a non-oxidation atmosphere. The reason is that 1000°C corresponds to a temperature at which a volatile material in an organic binder is sufficiently released to complete a carbon-based binding structure so that stable compressibility and bonding capability are exhibited, and a temperature at which the reaction to deposit of the specific metal is initiated. [0099] If the content of the specific metal in the remainder is greater than 15 mass%, the structural strength and the bonding capability of the intermediate layer are enhanced. On the other hand, the binding structure based on the metal carbide excessively increases the strength of the refractory structure of the intermediate layer, which is likely to spoil compressibility of the intermediate layer to cause difficulty in obtaining required compressibility. Moreover, the specific metal is melted during a course of temperature rise to cause a risk of flowing out from an original position in the matrix, which is likely to cause difficulty in uniformly obtaining the structural strength and the bonding force over the intermediate layer. This also leads to breakup of a part of the matrix structure and formation of an interlayer gap, and molten steel and other foreign substances are likely to intrude into a resulting void or the like. If the content of the specific metal in the remainder is less than 0.5 mass%, the effect of enhancing the structural strength of the intermediate layer and increasing the bonding strength in a non-oxidation atmosphere at 1000°C to 0.01 MPa or more cannot be obtained, and segregation is likely to occur. Moreover, this is likely to lead to breaking or peel-off of the intermediate layer and intrusion of molten steel and other foreign substances. [0100] The specific metal is limited to at least one of Al, Si and Mg for the following reason. In the specific metal, Al or Mg has a high affinity with oxygen, and an ability to capture oxygen to form a deposit excellent in corrosion resistance, such as AI2O3 or MgO. Further, Si reacts with carbon in the intermediate layer at a high-temperature region of about 1300°C or more to form SiC excellent in corrosion resistance. Preferably, a purity of the specific metal is maximized in view of reactivity and dispersibility. However, as long as the reactivity is impaired, the purity of the specific metal may be lowered (any metal or alloy sold on the market (industrially produced and generally distributed) with a label indicating that a primary component consists of the specific metal). [0101] Preferably, a particle size of the specific metal is minimized in view of reactivity and dispersibility. However, as the particle size becomes smaller, a handling risk becomes higher, and oxidation in air is more likely to occur. Thus, preferably, a lower limit and an upper limit of the particle size are set to about 5 um, and about 300 um, respectively. More preferably, the particle size is set to 20 um or less, because, when the particle size is set to 20 um or less, a surface area is sharply increased to provide enhanced reactivity and more enhanced dispersibility. [0102] As a percentage with respect to a total amount of the remainder in the intermediate layer, a carbon component to be reacted with the specific metal is essentially contained in an amount of 15to99.5mass%. [0103] A carbon source may be: a thermosetting resin, such as a phenol resin which leaves carbon during heating-up; various types of pitches; carbon black; graphite and carbon fiber. A combination of two or more of these materials may also be used. Preferably, the carbon source contains: carbon having a minimized particle size, such as carbon black; or amorphous carbon derived from the binding structure (hereinafter referred to simply as "fine carbon") to enhance reactivity with the specific metal, and uniformity. Further, an organic adhesive or resin, such as polyvinyl acetate-based resin, epoxy resin, acrylic resin or polyester resin, may be used in order to impart structural strength in a temperature range of room temperature to about several hundred °C. [0104] Preferably, the matrix structure of the refractory material of the intermediate layer further contains a base material for forming a backbone of the continuous binding structure and the matrix structure, such as graphite or carbon fiber (hereinafter referred to simply as "backbone carbon"), in addition to the fine carbon providing fundamental structural strength and bonding force. In particular, graphite and carbon fiber are preferable, because graphite can provide a flexible and continuous three-dimensional structure by taking advantage of a layered crystal structure and a flat particle shape thereof, and carbon fiber can also provide the same three-dimensional structure. [0105] The flexible and continuous three-dimensional structure can be formed in the matrix in the above manner. In this case, toughness can also be imparted to the binding structure including carbide after reaction with the specific metal, to suppress breakup of a portion of the matrix therearound which would otherwise occur when the hollow refractory aggregate is deformed or broken by stress, so as to further enhance soundness as a layer. [0106] In regard to a specific rate of the fine carbon and the backbone carbon, the backbone carbon, such as graphite or carbon fiber, having a large aspect ratio and an effect of enhancing three-dimensional continuity, is contained, as a percentage with respect to the total carbon amount of 15 to 99.5 mass%, in an amount of 70 to 95 mass%. If the content is less than 70 mass%, the three-dimensional continuity will deteriorate to cause a risk of spoiling flexibility. If the content is greater than 95 mass%, the bonding strength is limited to a low level to cause a risk of the occurrence of local breaking. [0107] As a method of assistively accelerating the enhancement in corrosion resistance by the effect of the deposit of the specific metal on an operation surface based on the coexistence of the specific metal and carbon, there is a technique of incorporating a refractory raw material excellent in corrosion resistance as a component of the remainder in combination. However, in the case where a refractory raw material consisting of a component other than the hollow refractory aggregate is contained in a part of the remainder other than the specific metal and carbon (the refractory raw material will hereinafter be also referred to as "additional component"), it is necessary to select, as an additional component, a refractory aggregate comprising a primary component free of a low-temperature melting phenomenon and a volatilization/vanishment phenomenon at a casting temperature in relation to the inner bore-side layer and the outer periphery-side layer. If a liquid phase is created at the casting temperature due to contact with the inner bore-side layer and the outer periphery-side layer, the bonding strength in a hot condition will be undesirably reduced, and the structural strength of the refractory material of the intermediate layer will be undesirably increased to a level spoiling compressibility, for example, due to excessive sintering. The inside of the refractory material of the intermediate layer is exposed to a strong reduction atmosphere. Thus, if the additional component is a highly volatile component, such as a SiC>2 component, which is not formed as a mineral substance, the additional component itself will be undesirably vaporized and vanished, while causing vanishment of a carbon component. [0108] An aggregate selectable for the additional component includes AI2O3, MgO, Z1O2 and AI2O3 • MgO-based spinel. The additional component may be appropriately selected from these components in conformity to a material of the inner bore-side layer to prevent a contact portion between the intermediate layer and the inner bore-side layer from creating a low-melting-point substance or the like. For example, when the inner bore-side layer consists of a CaO-containing refractory material, an MgO-based refractory aggregate is suitable. When the inner bore-side layer primarily comprises an AI2O3 or MgO-based material, AI2O3, MgO or AI2O3 • MgO-based spinel is suitable. Preferably, the additional component comprises an MgO-based refractory aggregate containing MgO at a purity of 90% or more. In this case, the additional component is suitable when the inner bore-side layer consists of an Al203-based material or a Zr02-based material, and desirably capable of widely coping with various inner bore-side layers. [0109] In case where a lower limit of a thickness of the intermediate layer is set to 1 mm, a particle size of the refractory aggregate constituting the additional component is preferably set to 0.5 mm or less to enhance dispersibility, and uniformity in the aforementioned various functions of the intermediate layer. [0110] The intermediate layer is required to have corrosion resistance in such a case that, when a deficient portion is formed in the inner bore-side layer due to various actions during casting operation, it is necessary to suppress or prevent molten steel and other foreign substances from being brought into direct contact with the outer periphery-side layer having lower corrosion resistance, and it is necessary for the intermediate layer itself to reliably have durability, such as corrosion resistance and erosion resistance. [0111] In the continuous casting nozzle, a portion to be directly exposed to molten steel, such as a deficient portion of the inner bore-side layer due to damage, a locally damaged portion in a weak region (e.g., an interface region between the inner bore-side layer and the nozzle body (outer periphery-side layer), a gas pool for gas injection or an interlayer junction), such as an interface region between the inner bore-side layer and the nozzle body (outer periphery-side layer), or an outlet portion of an immersion nozzle, exists or is likely to exist even in the form of a workpiece during a course of a production process thereof or a unused product. If the portion to be directly exposed to molten steel is poor in durability, such as corrosion resistance and erosion resistance, molten steel will intrude between the inner bore-side and outer periphery-side layers, for example, due to a selective vanishment of the portion, to case a defect of the continuous casting nozzle fatal to continuous casting operation, such as breaking of the continuous casting nozzle. [0112] The refractory material for the intermediate layer of the present invention is suitable for a continuous casting nozzle having a function or structure for allowing molten steel to pass through an inner bore thereof, such as an immersion nozzle, an open nozzle, a long nozzle for a ladle, a sliding nozzle (hereinafter referred to as "SN"), an SN upper nozzle, or an SN lower nozzle (also called "collector"). [0113] A material for the inner bore-side layer of the continuous casting nozzle is not limited to a specific type, but a refractory material, such as an Al203-based material, an MgO-based material or a Zr02-based material, having properties required for the continuous casting nozzle depending on each continuous casting operation, specifically, such as erosion (abrasion) resistance of a portion for contact with molten metal, corrosion resistance of an inner bore, anti-attachment of inclusions such as AI2O3, onto an inner bore, and suitable for each intended purpose, may be used on a case-by-case basis (the refractory material may contain graphite and other component). Similarly, a material for the outer periphery-side layer of the continuous casting nozzle is not limited to a specific type. The outer periphery-side layer typically serves as a nozzle body of the continuous casting nozzle. Thus, a part or entirety of a portion of the outer periphery-side layer may be made of a refractory material, such as a conventional AbOs-graphite based material, and a part or entirety of the mold-power portion may be made of a refractory material, such as a ZrCVbased material. [0114] The refractory material for the intermediate layer of the present invention is suitably used, particularly, in a combination of inner bore-side and outer periphery-side layers, wherein a refractory material of the inner bore-side layer has a thermal expansion coefficient greater than that of a refractory material of the outer periphery-side layer. It is understood that the refractory material for the intermediate layer of the present invention may also be used in case where, although each of inner bore-side and outer periphery-side layers has the same thermal expansion coefficient, for example, because they are made of the same material, a temperature gradient in a continuous casting nozzle formed by the inner bore-side and outer periphery-side layers or thermal shock to be supplied to the continuous casting nozzle is large enough to cause breaking of the continuous casting nozzle. [0115] Based on the above knowledge about the intermediate layer, the inventors have further found that a unique condition exists when a CaO-MgO based refractory material is provided as an inner bore-side layer. [0116] As disclosed, for example, in JP 2003-320444A, a CaO-MgO based refractory material is provided as an inner bore-side layer of a continuous casting nozzle to allow conventional problems, particularly, a problem of attachment of inclusions (typically, alumina) onto a surface of an inner bore and clogging of the inner bore, to be solved. However, the technique of employing the CaO-MgO based refractory material as an inner bore-side layer gave rise to new problems, particularly, a problem of breaking of an outer periphery-side layer due to expansion splitting, melting loss, breaking or peel-off of the inner bore-side layer, intrusion of scull between the inner bore-side layer and the outer periphery-side layer, or breaking of various regions of the nozzle. [0117] Therefore, in the present invention, for the continuous casting nozzle where the inner bore-side layer is made of an MgO-CaO based refractory material, measures are taken to prevent damage of the outer periphery-side layer due to thermal expansion of the inner bore-side layer, and maintain fixation with each of the inner bore-side layer and the outer periphery-side layer without formation of a void space between the inner bore-side and outer periphery-side layers causing intrusion of molten metal. [0118] Firstly, in the present invention, a composition of a CaO-MgO based material to be provided as an inner bore-side layer is specified. Specifically, a CaO-MgO based material containing a CaO component and an MgO component in a total amount of 80 mass% or more, wherein a mass ratio of CaO to MgO (CaO/MgO) is in the range of 0.2 to 1.5, is provided as an inner bore-side layer. [0119] Thus, the inner bore-side layer having a CaO component-based function of maintaining attachment resistance and an MgO component-based function of maintaining corrosion resistance in a balanced manner is provided to sufficiently bring out an anti-alumina clogging function. [0120] The CaO component reacts with an in-steel alumina-based deoxidation product which comes on a molten steel stream and into contact with a surface of an inner bore of the continuous casting nozzle, to create a CaO-AkOa based low-melting-point substance on a contact interface. This allows a slagged reaction product to easily flow down into a mold on the molten steel stream so as to prevent an alumina-clogging phenomenon in the nozzle. On the other hand, when an amount of the CaO component is increased, the CaO component will be continuously supplied from the refractory material to molten steel, so that an amount of melting loss in the refractory material is increased, and an amount of inclusions in steel is increased to cause deterioration in steel quality. [0121] Although the MgO component is advantageous in terms of melting-loss resistance, because it does not create any low-melting-point substance through a reaction with an alumina component, an increase in amount of the MgO component is disadvantageous in terms of the alumina-clogging phenomenon. [0122] Thus, the mass ratio CaO/MgO and the total amount (CaO + MgO) are critical parameters having an impact on melting-loss resistance and alumina-attachment resistance. Further, in terms of continuous casting operation, a flow rate of molten steel and an amount of alumina contained in molten steel have an impact on the melting-loss resistance and the alumina-attachment resistance. Generally, as the molten-steel flow rate becomes higher, an amount of alumina attachment becomes smaller and an amount of melting loss becomes larger. Further, as a concentration of in-steel alumina becomes higher, alumina attachment is accelerated under a certain condition. In conclusion, it is necessary to perform a material design in a composition range capable of achieving a balance between the alumina-attachment resistance and the melting-loss resistance while taking into account the conditions of casting operation and the type of molten steel. [0123] In accordance with the above requirements, in the present invention, the composition of the inner bore-side layer is specified as described above. Specifically, if the mass ratio of the CaO component to the MgO component (CaO/MgO) is less than 0.2, the CaO component cannot be continuously supplied from the inner bore-side layer under typical conditions of casting operation where the molten-steel flow rate is 5t/min or less, which precludes a possibility to maintain the alumina-attachment resistance. If the mass ratio CaO/MgO is greater than 1.5, the supply of the CaO component from the inner bore-side layer will be excessively increased to cause an increase in amount of melting loss in the inner bore-side layer itself, and an increase in amount of in-steel inclusions. The total amount of the CaO component and the MgO component is set to 80 mass% or more so as to achieve a balance between corrosion resistance and melting-loss resistance. [0124] Preferably, the remainder after excluding the CaO component and the MgO component is comprised of a refractory material other than the CaO component and the MgO component, particularly, a carbon-based refractory material, in view of maintaining a balance between the corrosion resistance and the melting-loss resistance (anti-attachment). In the case where a carbon-based refractory material is used as the reminder, if the total amount of the CaO component and the MgO component is less than 80 mass%, an amount of a carbon component in the reminder is excessively increased, and thereby melting of the carbon component into molten steel becomes prominent to cause a problem that an amount of melting loss in the inner bore-side layer is excessively increased to shorten a lifetime of the nozzle, and an amount of in-steel inclusions is increased. [0125] A CaO component source and an MgO component source to be used in the refractory material of the inner bore-side layer may be a dolomite clinker, a synthetic dolomite raw material, a magnesia raw material, or a calcia raw material. In particular, a CaO component in a burnt dolomite clinker continuously exists in the clinker. Thus, the burnt dolomite clinker is desirable in terms of continuous supply of CaO. [0126] Preferably, a particle size of the CaO and MgO components is set in the range of 0.1 to 3 mm. If the particle size is less than 0.1 mm, a hydration phenomenon is likely to occur to cause problems in quality stability and volume stability, for example, in case where an MgO-CaO-based fine power is used in a large amount. If the particle size is greater than 3 mm, nonuniformity in composition or particle size in a shaped body is likely to occur, which is undesirable in terms of uniformity. [0127] An intermediate layer for use with the above inner bore-side layer made of a CaO-MgO based material is prepared in such a manner that, after a heat treatment in a non-oxidation atmosphere at a temperature of about 600°C, it contains a hollow refractory aggregate in an amount of 10 to 75 volume%, with the remainder which contains, on an assumption that a total amount of the remainder is 100 mass%, one or more selected from the group consisting of Al, Si, Mg and an alloy comprising any combination of two or more thereof, in a total amount of 0.5 to 15 mass% in terms of only a content of the metals, and carbon in an amount of 2 to 99.5 mass%, wherein a value derived by dividing a mass ratio of CaO contained in the inner bore-side layer to the entire inner bore-side layer by a mass ratio of a total of AI2O3, SiC>2 and an alkali metal oxide contained in the intermediate layer to the entire intermediate layer is 10 or more, as mentioned above. [0128] The reason is as follows. If the CaO component in the inner bore-side layer which contains the CaO component and the MgO component in a total amount 80 mass% or more, wherein the mass ratio of CaO to MgO (CaO/MgO) is in the range of 0.2 to 1.5, comes into contact with a large amount of AI2O3 and Si02 components, a CaO-Al203-Si02 based reaction product will be created, particularly, in prolonged casting operation, wherein the CaO component in the inner bore-side layer is consumed during the reaction, so that a function of capturing an AI2O3 inclusion in molten steel is lowered, and a bonded portion with the intermediate layer is excessively strengthened and deformed with shrinkage or the like to produce an uneven tension stress in the inner bore-side layer to increase a possibility to give rise to breaking (cracking) in the inner bore-side layer. [0129] When an alkali metal oxide is added to the Al2O3-SiO2 based component, the above phenomenon is accelerated. Further, if the value derived by dividing a mass ratio of CaO contained in the inner bore-side layer to the entire inner bore-side layer by a mass ratio of a total of AI2O3, SiO2 and an alkali metal oxide contained in the intermediate layer to the entire intermediate layer is less than 10, the phenomenon becomes prominent. [0130] In view of providing enhanced corrosion resistance against molten steel to an intermediate layer for use with the inner bore-side layer made of a CaO-MgO based material, an MgO-based refractory aggregate or an AI2O3 • MgO-based spinel refractory aggregate is suitably used as refractory aggregate particles as the remaining component of the intermediate layer after excluding the hollow refractory aggregate, the carbon and the specific metal. Preferably, an amount of the MgO-based refractory aggregate or the AI2O3 • MgO-based spinel refractory aggregate to be contained in the remaining component is adjusted to be 50 mass% or more (including 100 mass%). [0131] A first reason is to provide a material combination which is less likely to produce an excessive cross-reaction, such as sintering or melting, in an interface between the inner bore-side and outer periphery-side layers. For the inner bore-side layer which contains the CaO component and the MgO component in a total amount 80 mass% or more, wherein the mass ratio of CaO to MgO (CaO/MgO) is in the range of 0.2 to 1.5, a magnesia or spinel (spinel comprising AI2O3 and MgO) based refractory aggregate itself or a mixture comprising the refractory aggregate is optimal, because it is less likely to produce a cross-reaction with the above refractory material of the inner bore-side layer. [0132] A second reason is that the MgO-based refractory aggregate or the AI2O3 • MgO-based spinel refractory aggregate is less likely to produce a cross-reaction with an Al203-Si02-C, AI2O3-C, Zr02-C or MgO-C based refractory material commonly used as an outer periphery-side layer. [0133] A third reason is that, as compared with other aggregate, such as alumina-silica based refractory aggregate particles, MgO is relatively less likely to produce a cross-reaction with a vitreous component, a silica component, etc., in the hollow refractory aggregate. [0134] In the above case, the outer periphery-side layer (nozzle body) may be made of any one of an Al203-C-based refractory material, a ZrCh-C-based refractory material and an MgO-C-based refractory material, wherein a relationship between C and each of AI2O3, Z1O2 and MgO, such as a composition ratio and an existence form, is not particularly limited. EFFECT OF THE INVENTION [0135] As above, in a continuous casting nozzle, when an inner bore-side region has thermal expansion greater than that of an outer periphery-side region, and, particularly, when a highly-functional layer having high corrosion resistance, high attachment resistance, etc., is disposed on the side of an inner bore to enhance durability, the refractory material of the present invention is used for an intermediate layer of the continuous casting nozzle. This makes it possible to prevent expansion splitting of an outer periphery-side layer due to a difference in thermal expansion between an inner bore-side layer, and the outer periphery-side layer as a nozzle body, and prevent peel-off and breaking of the inner bore-side layer during casting operation. [0136] In addition, based on a deposit of the specific material, a stable bonding capability can be obtained, while achieving higher density and higher structural strength of the refractory material itself of the intermediate layer, so as to improve stability of a multi-layer structure and corrosion resistance of the intermediate layer. [0137] Furthermore, the present invention provides an inner bore-side layer having a significantly high effect of suppressing attachment of inclusions (typically, AI2O3) onto the inner bore. Further, in the use of such an inner bore-side layer, the present invention makes it possible to prevent the occurrence of excessively strong bonding between the inner bore-side layer and the intermediate layer due to a CaO component, and solve a problem causing breaking (cracking) of the inner bore-side layer, such as drop-off or displacement of the inner bore-side layer due to insufficient bonding, to allow continuous casting operation to be stably performed over a long period of time. [0138] In the continuous casting nozzle of the present invention, various refractory materials having properties required for the continuous casting nozzle depending on unique conditions of each continuous casting operation, specifically, such as erosion (abrasion) resistance of a portion for contact with molten metal, corrosion resistance of an inner bore, anti-attachment of inclusions such as AI2O3, onto an inner bore, and suitable for each intended purpose, can be used for each region on a case-by-case basis, so that the number of selectable materials and combinations thereof can be significantly increased. This also contributes to extension of a lifetime of a continuous casting nozzle, enhancement of steel quality, stable casting operation, and resource saving. 1 BRIEF DESCRIPTION OF THE DRAWINGS [0139] FIG. 1 is a sectional view of an immersion nozzle as one example of a continuous casting nozzle using a refractory material for an intermediate layer of the present invention, taken along an axis thereof. FIG. 2 is a schematic sectional view showing a sample and a testing apparatus during a bonding strength test, taking along an axis of the sample. FIG. 3 is a schematic vertical sectional view showing a sample and a testing apparatus during a breaking test for a hollow refractory aggregate in Example A. BEST MODE FOR CARRYING OUT THE INVENTION [0140] A method of producing a refractory material of the present invention will be firstly described. In case where the refractory material itself of the present invention comprises a hollow refractory aggregate, carbon, and a specific metal, refractory aggregate particles constituting these components are mixed together. Then, a binder, such as an organic resin (e.g., phenol resin or vinyl acetate resin) which is capable of wetting the obtained power mixture to provide aggregability of the respective particles or bonding capability, and, after curing, exhibiting structural strength enough to ensure shape retainability as a shaped body, is added to the powder mixture in an appropriate amount required for shaping and shape retention, and they are kneaded to obtain a kneaded mixture. Then, the kneaded mixture is filled in a pre-defined space by an appropriate technique, such as pouring or injection, and shaped. Subsequently, the obtained shaped body is subjected to a heating treatment, such as drying or burning, at a temperature of about 110 to 600°C, at an appropriate temperature depending on properties of the binder and others to obtain the refractory material. Details of the method will be described below. [0141] 10 to 75 volume% of hollow refractory aggregate is mixed with 25 to 90 volume% of carbonaceous particles derived from flaky graphite, amorphous or earthy graphite, carbon black, pitch, resin or the like, and oxide particles or metal particles, such as magnesia, zirconia or corundum-based particles. [0142] The 25 to 90 volume% of carbonaceous particles derived from flaky graphite, amorphous or earthy graphite, carbon black, pitch, resin or the like, and oxide particles or metal particles, are combined and mixed together, in such a manner that the obtained mixture comprises, as a percentage with respect to a total amount of the remainder after excluding the hollow refractory aggregate, one or more (specific metal) selected from the group consisting of Al, Si, Mg and an alloy comprising any combination of two or more thereof, in a total amount of 0.5 to 15 mass% in terms of only a content of the metals, and carbon in an amount of 2 to 99.5 mass%, with the remainder (including zero) being a refractory aggregate other than the specific metal and carbon. Preferably, a maximum particle size of this raw material is set to 0.5 mm or less to allow the refractory material of the intermediate layer to have uniform compressibility while being formed in mortar form excellent in working efficiency during application. [0143] An amount of the hollow refractory aggregate may be determined by calculating a required compressive rate from a relationship between a thickness of the refractory material of the intermediate layer and each of thermal expansion coefficients of an inner bore-side layer and an outer periphery-side layer, and adjusting a ratio between the hollow refractory aggregate and the remaining raw materials. [0144] Further, a mixing rate of each of the hollow refractory aggregate and the remaining raw materials, i.e., the remainder such as the refractory aggregate may be adjusted in such a manner that a value derived by dividing a mass ratio of CaO contained in the inner bore-side layer to be combined with the intermediate layer, to the entire inner bore-side layer, by a mass ratio of a total of AI2O3, SiCh and an alkali metal oxide contained in the intermediate layer to the entire intermediate layer is set to 10 or more. [0145] Then, a binder, such as an organic resin (e.g., phenol resin or vinyl acetate resin) which is capable of wetting the mixture to provide aggregability of the respective particles or bonding capability, and, after curing, exhibiting structural strength enough to ensure shape retainability as a shaped body, is added to the above powder mixture in an amount adjusted to provide softness suitable for shaping of the mixture, and they are kneaded using a mixer, such as a mortar mixer, to obtain a mortar mixture. On an assumption that the powder mixture is 100 mass parts, an amount of phenol resin or other organic resin to be used may be adjusted in the range of 40 to 90 mass parts depending on required working efficiency. [0146] Then, the mortar mixture is filled in a space pre-defined between the inner bore-side and outer periphery-side layers, by applying the mortar mixture onto one or both of surfaces of the inner bore-side and outer periphery-side layers and fitting the inner bore-side layer into the outer periphery-side layer, or other appropriate technique, such pouring or spraying, to integrate the inner bore-side layer and the outer periphery-side layer together. Then, the filled mortar mixture is subjected to a heating treatment, such as drying or burning, at a temperature of about 110 to 600°C, depending on properties of the binder and others, to allow the mortar mixture to exhibit shape retainability and interlayer fixing capability. [0147] Practically, the above process for the refractory material of the intermediate layer is mostly incorporated as a part of an after-mentioned production process for a continuous casting nozzle structure to provide a continuous casting nozzle in the form of a single piece of product. Alternatively, the refractory material may be formed as a member having any suitable shape, such as a tubular shape, and be assembled as a part of a continuous casting nozzle. In this case, the mortal mixture may be shaped using a mold form, and dried or burned in a non-oxidation atmosphere to form the refractory member. [0148] A production method for a continuous casting nozzle using the above refractory material of the intermediate layer will be described below. [0149] An inner bore-side layer is formed as a single piece of refractory shaped body, separately from a nozzle body of a continuous casting nozzle. As long as this inner bore-side layer is prepared as a refractory shaped body in advance, a production method therefor is not particularly limited. A specific example will be described below, wherein the inner bore-side layer contains a CaO component and an MgO component. [0150] A content rate of each component in a refractory raw material containing a CaO component and an MgO component, such as a fine powder raw material of burnt dolomite and a fine powder raw material of MgO clinker is adjusted in such a manner that, after a refractory shaped body of the refractory raw material is subjected to a heat treatment in a non-oxidation atmosphere at 600°C, the refractory shaped body contains a CaO composition and an MgO composition in a total amount of 80 mass% or more, wherein a mass ratio of CaO to MgO (CaO/MgO) is in the range of 0.2 to 1.5. Then, a binder, such as phenol resin, which has a binding capability after a heat treatment in a non-oxidation atmosphere at 600°C, and a liquid shaping assistant for obtaining a wet state suitable for shaping (if the binder is in liquid form, it can also serve as the shaping assistant) is added to the fine powder raw materials, and they are uniformly mixed together by a mixer to obtained a mixture for shaping. [0151] The obtained mixture is shaped by an appropriate forming machine, such as a CIP (Cold Isostatic Press) machine, a hydraulic press machine or a friction press machine, and the obtained shaped body is dried at a temperature of about 150°C or more, or subjected to a heat treatment in a non-oxidation atmosphere. Subsequently, according to need, the shaped body, such as an outer peripheral surface thereof, is machined into a shape suitable for being attached to the nozzle body prepared as a separate body, by a conventional machining technique. A generally required treatment for the raw materials or the shaped body, such as measures against hydration, may be performed depending on the raw materials and production conditions on a case-by-case basis. [0152] A space having a given thickness for the intermediate layer is pre-defined between the shaped body separately prepared as the inner bore-side layer in the above manner and the nozzle body separately prepared as the outer periphery-side layer, and the refractory material of the present invention is filled into the space to form the intermediate layer so as to complete a continuous casting nozzle having a multi-layer structure. [0153] The refractory material of the present invention to be used for the intermediate layer is formed in muddy unshaped form enough to be able to be filled into a narrow space between the inner bore-side and outer periphery-side layers. With a view to providing working efficiency during filling, a liquid resin may be added to the refractory material in an amount of about 40 to 90 mass parts (this amount is determined in consideration of a volume of the space and working efficiency during setup) with respect to and in addition to 100 mass parts of the powder mixture comprising the hollow refractory aggregate, the carbon raw material in solid form, and the remaining components including the refractory aggregate, and they are kneaded. [0154] The refractory material for the intermediate layer improved in working efficiency during setup is applied onto an outer peripheral surface of the inner bore-side layer provided with a spacer for defining a given space having a thickness of the intermediate layer, or an inner peripheral surface of the outer periphery-side layer, and then the inner bore-side layer is inserted into the outer periphery-side layer (nozzle body of a continuous casting nozzle to be produced). In this state, a space defined between the outer peripheral surface of the inner bore-side layer and the inner peripheral surface of the outer periphery-side layer becomes equal to the thickness of the intermediate layer. [0155] In place of the above filling technique based on application, the refractory material for the intermediate layer may be prepared to have a higher fluidity, for example, by adding liquid at a higher rate, and poured into a given space defined between the inner bore-side and outer periphery-side layers. [0156] An obtained continuous casting nozzle having the filled refractory material for the intermediate layer is subjected to a heating treatment, such as drying or burning, to cure the refractory material for the intermediate layer so as to fix the inner bore-side layer to the outer periphery-side layer. The curing may be performed at an appropriate temperature, in a temperature range of room temperature to about 600°C, depending on properties of the binder contained in the refractory material of the intermediate layer. For example, in case where a vinyl-based resin is used as the binder, the curing may be achieved by drying at about 150°C. In case where a phenol resin is used as the binder, a heating temperature is preferably set at 200°C or more. Subsequently, the continuous casting nozzle may further be burnt, for example, in a non-oxidation atmosphere at a temperature of about 1000 to 1300°C. In the above manner, a semifinished product of the continuous casting nozzle of the present invention can be obtained. [0157] The hollow refractory aggregate used in the refractory material for the intermediate layer is kept from broking by an external force during installation/setup of the inner bore-side layer to the outer periphery-side layer, to prevent the occurrence of an undesirable situation where a thickness of the intermediate layer becomes excessively reduced due to the setup operation, or a required compressibility of the intermediate layer is spoiled due to absorption of a solvent. Further, the hollow refractory aggregate is formed in a balloon-like configuration which has a rounded outer shape almost without an edge as in a spalled particle. This makes it possible to obtain an advantage of being able to improve fluidity of the muddy refractory material of the intermediate layer, i.e., to reduce an amount of liquid phase so as to achieve a dense matrix structure. [0158] However, in either technique, if the hollow refractory aggregate is pressed by a pressure greater than the structural strength thereof during shaping or setup operation, it will be broken to lose a stress relaxation capability. Thus, the hollow refractory aggregate cannot be subjected to various high-pressure press forming process commonly used for simultaneously/integrally forming a plurality of layers of a continuous casting nozzle, such as cold isostatic press (OP), based on a pressure fairly greater than at least 2.5 MPa at which the hollow refractory aggregate is broken. [0159] In the above production method, a binder is used in the refractory material for the intermediate layer to provide a shape retainability of the intermediate layer itself and a structural strength of the intermediate layer between room temperature to a hot condition during use, and ensure a formability of the mixture. In case where the inner bore-side layer contains an MgO-CaO based component, particularly CaO existing by itself (not in the form of a solid solution or a compound), it is necessary to use a binder which is devoid of water, and less likely to release moisture during heating, in order to prevent breakup or the like of the inner bore-side layer after setup, due to hydration of a CaO component therein. A binder meeting this requirement includes non-water based phenol resin, non-water based furan resin, tar, melamine resin, epoxy resin, and polyvinyl acetate resin using alcohol as a solvent. [0160] Carbon derived from the binder and left at 600°C or more serves as a part of a carbon component of the refractory material of the intermediate layer. [0161] The semifinished product of the continuous casting nozzle after the filling and the heating treatment may be subjected to the same process as that for conventional continuous casting nozzle, such as machining of an outer periphery thereof and others, and application of an antioxidant. [0162] Through the above production method, the continuous casting nozzle of the present invention comprising the intermediate layer with compressibility and the inner bore-side and outer periphery-side layers continuously integrated with the intermediate layer can be obtained. [0163] FIG. 1 shows an immersion nozzle as one example of the continuous casting nozzle of the present invention. In FIG. 1, the reference numerals 1, 2, 3 and 4 indicate an intermediate layer, an inner bore-side layer made of an MgO-CaO-based refractory material, an alumina-graphite layer as a part of an outer periphery-side layer serving as a nozzle body of the continuous casting nozzle, and a zirconia-graphite based layer serving as a powder region which is remaining part of the outer periphery-side layer, respectively. Further, the reference numerals 5,6 and 7 indicate an inner bore, a molten-steel inlet opening, and an outlet opening. [EXAMPLE] [0164] Various examples will be described below. [0165] « Example A is a result of a test for checking an influence of an average radius R of each particle of a hollow refractory aggregate and a ratio of the average radius R to an average wall thickness t of the particle (R/t) on breaking of the hollow refractory aggregate when an external force of 2.5 MPa is applied thereto. [0166] Table 1 shows a material/structure and a test result of each sample in Example A. [0167] A hollow refractory aggregate for test samples was obtained by selecting one of available products generally sold on the market, dispersing particles thereof in water, collecting floated ones of the particles, classifying the collected particles, and drying the classified particles at 110°C. The refractory aggregate for test samples had a composition which comprises 70 mass% or more of SiC>2, 1 to 10 mass% of an alkali metal oxide and an alkaline earth metal oxide in total, and 5 to 20 mass% of AI2O3, and a glassy structure. [0168] The refractory aggregate for test samples had three types of particles having average radii of 2.5 um (desirable minimum radius), 250 um (desirable maximum radius) and 35 um (intermediate radius). Each type of particles was classified of into a plurality of groups each having a different average wall thickness to obtain a plurality of test samples each having a different ratio R/t. [0169] In the test, as shown in FIG. 3, each of the test samples 10 was filled in a cylindrical metal container 11 having an inner diameter of 60 mm to have an initial height dimension (thickness) of 10 mm, and pressed by a pressure of 2.5 MPa using a press machine (having an upper liner 12 and a lower liner 13) until it is stopped. Then, after taking the test sample out from the container 11, particles of the test sample were dispersed in 1 liter of water to separate the particles into a floated group and a precipitated group, and the floated particles are collected. After drying the collected particles, a weight of the dried particles was measured. [0170] A breakup rate (%) was derived by subtracting a total weight of the floated particles from a total weight of the test sample 10 initially filled in the cylindrical metal container 11 (this total weight will hereinafter be referred to as "initial total weight") and dividing the obtained value by the initial total weight, and expressed by percentage. [0171] In Example A, considering that a matrix portion exhibits a certain level of compressibility, a requirement for obtaining a required compressive rate was set as a condition that the breakup rate of the hollow refractory aggregate is 90% or more. Further, in this test, it is assumed that fragments of the particles broken by the pressing are filled in an inter-particle space to bring out a stress relaxation capability by themselves, whereby the remaining unbroken particles become less likely to be broken, and a part of the unbroken particles will be left without breaking. Thus, it can be evaluated that a group of particles having a breakup rate of 90% or more exhibits the same or better level of breaking capability in a refractory structure. [0172] Each of the samples having an average radius R ranging from the desirable minimum radius 2.5 urn to the desirable maximum radius 250 um had a breakup rate of 90% or more when the ratio R/t is 10 or more. [0173] Example B is a result of a test for checking an influence of a volume percentage of a hollow refractory aggregate with respect to a refractory material, on a compressibility and a bonding strength, and a result of a simulation test for casting of molten steel based on inner-bore heating. [0174] Table 2 shows a composition and a test result of each sample in Example B. [0175] A hollow refractory aggregate used in Example B was a powder (sample 3) which has the same composition as that of the hollow refractory aggregate used in Example A, an average diameter R of 35 urn, an average wall thickness t of 1 um, and a breakup rate of 99% at 2.5 MPa. Any sample in Example B was prepared to have the same composition of the remainder after excluding the hollow refractory aggregate. [0176] A compressive rate was measured as follows. Two bonding-target test pieces having a size of 20 mm q> x 50 mm L and comprising about 75 mass% of AI2O3 and about 25 mass% of C were prepared by the same production method (the same conditions of forming pressure, drying, burning, etc.) as that for conventional continuous casting nozzles. Then, each of the samples formed as a mortar mixture was installed in a space between respective flat surfaces of the two bonding-target test pieces to have a thickness of 2 mm to form a measurement sample by the method described in the "MEANS FOR SOLVING THE PROBLEM", and the measurement sample was subjected to drying. A compressive rate of the obtained measurement sample was measured at 1000°C and 1500°C (in a nitrogen gas atmosphere). [0177] A bonding strength was measured as follows. Through the same production method (the same conditions of forming pressure, drying, burning, etc.) as that for conventional continuous casting nozzles, a cylindrical tube serving as an outer periphery-side layer made of a refractory material comprising about 55 mass% of AI2O3, about 30 mass% of C and about 14 mass% of Si02, which is commonly-used for a nozzle body of a continuous casting nozzle, and formed to have a size of 95 mm q> (inner diameter) * 100 mm L, and a cylindrical tube serving as an inner bore-side layer made of a dolomite-based refractory material comprising about 49 mass% of MgO, about 44 mass% of CaO and about 4 mass% of C and formed to have a size of 90 mm cp (outer diameter) x 100 mm L, were prepared. Then, each of the samples formed as a mortar mixture was installed in a space between the two cylindrical tubes to have a thickness of 2.5 mm, and subjected to drying to form a ring-shaped measurement sample. A bonding strength of the obtained ring-shaped measurement sample was measured at 1000°C and 1500°C (in a nitrogen gas atmosphere) by the method described in connection with FIG. 2. [0178] A cylindrical sample for the inner-bore heating test was prepared as follows. Firstly, a cylindrical tube-shaped body was formed by a CIP process. This shaped body was subjected to drying at 200°C and a heat treatment in a non-oxidation atmosphere at 1000°C, and then an outer periphery of the shaped body was machined to obtain a dolomite-carbon based sleeve having a size of 90 mm (p (outer diameter) x 70 mm

Example D is a result of a test for checking an influence of a percentage of carbon with respect to the remainder after excluding a hollow refractory aggregate, on a compressibility and a bonding strength, and a result of a simulation test for casting of molten steel based on inner-bore heating. [0189] Table 4 shows a composition and a test result of each sample in Example D. [0190] A hollow refractory aggregate used in Example D has the same composition and the same particle-size distribution as those of the hollow refractory aggregate used in each of Examples B andC. [0191] In Example D, fundamentally, (except the sample containing carbon in an amount of 99.5 mass% or more), an amount of a specific metal and a phenol resin solution (converted to C to be left after a heat treatment in a non-oxidation treatment at 1000°C) was fixed, and an amount of carbon was changed by replacing an MgO fine powder (MgO purity: 95 to 98 mass%; other Examples have the same condition) with a graphite fine powder. [0192] The measurement of a compressive rate, and a bonding strength, and the simulation test for casting of molten steel based on inner-bore heating, were carried out in the same manner as that in each of Examples B and C. [0193] As is evident from the measurement result shown in Table 4, the desired range of the bonding strength of 0.01 to 1.5 MPa can be obtained, when a content of carbon in the remainder metal is in the range of 2 to 99.5 mass%. [0194] In the inner-bore heating test, a desired result was obtained when the content of carbon is in the range of 2 to 99.5 mass%, and this tendency primarily corresponds to the given range of the bonding strength, as with Example C. [0195] Example E is a result of a test for checking an influence of a total amount of CaO and MgO and a mass ratio of CaO to MgO in a refractory material for an inner bore-side layer, on melting loss and alumina attachment. [0196] Each of various types of CaO-MgO based materials each having a different mass ratio of CaO/MgO and/or a different content of CaO + MgO was shaped by a cold isostatic press (CIP) process at 98 MPa, and the obtained shaped body was subjected to a heat treatment in a non-oxidation atmosphere. Then, a bar-shaped sample (20 x 20 x 160 mm) was cut out from the shaped body to obtain a test sample. [0197] Each of the samples was immersed in low-carbon aluminum-killed steel held at a temperature of 1550 to 1570°C, for 120 minutes, and then pulled up to measure a thickness of an alumina-attachment layer on a surface of the sample, and an amount of melting loss in the sample itself. For comparison, a conventional A^Oa-graphite based material was also subjected to the test. [0198] Table 5 shows a composition and a test result of each sample in Example E. [0199] In the samples 30 to 32, 34, 35, 37 and 38, each of the melting-loss amount and the alumina-attachment amount was in a well-balanced desired range. On the other hand, in the sample 28 comprising a conventional AG-based material, alumina attachment occurred although no melting-loss phenomenon occurred. The sample 28 is likely to cause a problem of clogging. In the sample 29 where the ratio of CaO/MgO is 1.7, large melting loss occurred. In the example 33 where the ratio of CaO/MgO is 0.1, large alumina attachment occurred. In each of the examples 36 and 39 where the total amount of CaO + MgO is 75 mass%, an amount of melting loss in the sample became larger due to an influence of carbon amount. [0200] Depending on conditions of casting using the continuous casting nozzle of the present invention, the physical properties of the sample 28,29, 33, 36 or 39 are likely to cause a problem in prolonged casting operation. Thus, it is desirable that an inner bore-side layer has the physical properties of the samples 30 to 32, 34, 35, 37 or 38. [0201] Example F is a result of a test for checking, under a condition that an inner bore-side layer contains a CaO composition and an MgO composition in a total amount of 80 mass% or more, wherein a mass ratio of CaO to MgO (CaO/MgO) is in the range of 0.2 to 1.5, an influence of a value derived by dividing a mass ratio of CaO contained in the inner bore-side layer to the entire inner bore-side layer by a mass ratio of a total of AI2O3, Si02 and an alkali metal oxide contained in an intermediate layer to the entire intermediate layer, on a bonding strength, and a result of a simulation test for casting of molten steel based on inner-bore heating. [0202] Table 6 shows a composition and a test result of each sample in Example F. [0203] The sample 31 in Table 5 comprising 50 mass% of CaO component and 45 mass% of MgO component, wherein the mass ratio of CaO to MgO (CaO/MgO) is 1.1, and the sample 32 in Table 5 comprising 16 mass% of CaO component and 79 mass% of MgO component, wherein the mass ratio of CaO to MgO (CaO/MgO) is 0.2, were used as a refractory material of an inner bore-side layer. [0204] A hollow refractory aggregate used in Example F has the same composition and the same partiele-size distribution as those of the hollow refractory aggregate used in each of Examples B toD. [0205] In regard to a refractory material of the intermediate layer, based on the composition of the remainder (matrix) after excluding the hollow refractory aggregate, in the sample 7, the total amount of AI2O3, Si02 and an alkali metal oxide was adjusted primarily by changing a content of the hollow refractory aggregate. [0206] The measurement of a bonding strength, and the simulation test for casting of molten steel based on inner-bore heating, were carried out in the same manner as that in each of Examples B toD. [0207] As is evident from the result of this test, each of the samples 41 to 45, 47 and 48 where the value derived by dividing a mass ratio of CaO contained in the inner bore-side layer to the entire inner bore-side layer by a mass ratio of a total of AI2O3, Si02 and an alkali metal oxide contained in an intermediate layer to the entire intermediate layer (the value will hereinafter be referred to as "ratio C/I") is 10 or more, satisfies a requirement that the bonding strength is in the range of 0.1tol.5MPa. [0208] In the simulation test for casting of molten steel based on inner-bore heating, each of the above samples had a desired result. [0209] In contrast, the samples 40 and 46 where the ratio C/I is less than 10, had a bonding strength of less than 0.01 MPa at 1500°C, i.e., could not satisfy the requirement that the bonding strength is 0.1 MPa or more. Moreover, in the simulation test for casting of molten steel based on inner-bore heating, drop-off occurred in the 2nd cycle. [0210] Depending on conditions of casting using the continuous casting nozzle of the present invention, the physical properties of the sample 40 or 46 are likely to cause a problem in prolonged casting operation. Thus, it is desirable that an inner bore-side layer has the physical properties of the samples 41 to 45,47 and 48. EXPLANATORY OF CODES [0211] 1: intermediate layer (a layer consisting of a refractory material for an intermediate layer of the present invention) 2: inner bore-side layer 3: alumina-graphite based layer constituting outer periphery-side layer and serving as nozzle body of continuous casting nozzle 4: zirconia-graphite based layer constituting outer periphery-side layer and serving as powder region of continuous casting nozzle 5: inner bore 6: molten-steel inlet opening 7: outlet opening 8: table 9: crosshead 10: test sample (hollow refractory aggregate) 11: container 12: upper liner (jig for pressing based on downward movement) 13: lower liner (jig for pressing based on upward movement) 1. A refractory material for an intermediate layer of a continuous casting nozzle, which contains a hollow refractory aggregate in an amount of 10 to 75 volume%, wherein a ratio of an average radius R of each particle of the aggregate to an average wall thickness t of the particle satisfies the following relation: R/t > 10. 2. The refractory material as defined in claim 1, wherein the hollow refractory aggregate has a glassy structure which contains SiC>2 in an amount of 70 mass% or more, and an alkali metal oxide and an alkaline earth metal oxide in a total amount of 1 to 10 mass%. 3. The refractory material as defined in claim 1 or 2, which contains, as a percentage with respect to a total amount of the remainder after excluding the hollow refractory aggregate, one or more selected from the group consisting of Al, Si, Mg and an alloy comprising any combination of two or more thereof, in a total amount of 0.5 to 15 mass% in terms of only a content of the metals, and carbon in an amount of 2 to 99.5 mass%. 4. The refractory material as defined in any one of claims 1 to 3, which has a compressive rate of 10 to 80% as measured under a pressure of 2.5 MPa. 5. The refractory material as defined in any one of claims 1 to 4, which has a bonding strength * of 0.01 to 1.5 MPa with respect to other refractory material for the continuous casting nozzle, as measured under a hot condition in a non-oxidation atmosphere at a temperature of 1000 to 1500°C. 6. A continuous casting nozzle comprising a tubular refractory structure which has an inner bore formed along an axial direction thereof to allow molten metal to pass therethrough, and includes an inner bore-side layer disposed to define the inner bore and an outer periphery-side layer disposed radially outward of the inner bore-side layer, wherein, in a part or entirety of the tubular refractory structure, a refractory material of the inner bore-side layer has a thermal expansion greater than that of a refractory material of the outer periphery-side layer, the continuous casting nozzle being characterized in that the inner bore-side layer and the outer periphery-side layer are mutually independent shaped bodies, wherein a first one of the shaped bodies serving as the inner bore-side layer is fixed to the other, second, shaped body serving as the outer periphery-side layer through an intermediate layer having compressibility, and wherein: a bonding strength between the intermediate layer and each of the first shaped body serving as the inner bore-side layer and the second shaped body serving as the outer periphery-side layer is in the range of 0.01 to 1.5 MPa, as measured under a hot condition in a non-oxidation atmosphere at a temperature of 1000 to 1500°C; and the intermediate layer has a compressive rate K (%) satisfying the following Formula 1, K > (Di x ai - Do x ao) / (2 x Tm) — Formula 1 wherein: Di is an outer diameter (mm) of the inner bore-side layer; Do is an inner diameter (mm) of the outer periphery-side layer; Tm is an initial thickness (mm) of the intermediate layer at room temperature; ai is a maximum thermal expansion coefficient (%) of the refractory material of the inner bore-side layer in a temperature range of room temperature to 1500°C; and ao is a thermal expansion coefficient (%) of the refractory material of the outer periphery-side layer at a temperature in an initial stage of passing of molten steel through the continuous casting nozzle. 7. The continuous casting nozzle as defined in claim 6, wherein the intermediate layer is made of the refractory material as defined in any one of claims 1 to 5. 8. The continuous casting nozzle as defined in claim 6, wherein: the intermediate layer contains a hollow refractory aggregate in an amount of 10 to 75 volume%, wherein a ratio of an average radius R of each particle of the hollow refractory aggregate to an average wall thickness t of the particle satisfies the following relation: R/t > 10, the intermediate layer further containing, as a percentage with respect to a total amount of the remainder after excluding the hollow refractory aggregate, one or more selected from the group consisting of Al, Si, Mg and an alloy comprising any combination of two or more thereof, in a total amount of 0.5 to 15 mass% in terms of only a content of the metals, and carbon in an amount of 2 to 99.5 mass%; and the inner bore-side layer contains a CaO composition and an MgO composition in a total amount of 80 mass% or more, wherein a mass ratio of CaO to MgO (CaO/MgO) is in the range of0.2tol.5, and wherein a value derived by dividing a mass ratio of CaO contained in the inner bore-side layer to the entire inner bore-side layer by a mass ratio of a total of AI2O3, Si02 and an alkali metal oxide contained in the intermediate layer to the entire intermediate layer is 10 or more. 9. The continuous casting nozzle as defined in claim 8, wherein the hollow refractory aggregate in the refractory material of the intermediate layer has a glassy structure which contains Si02 in an amount of 70 mass% or more, and an alkali metal oxide and an alkaline earth metal oxide in a total amount of 1 to 10 mass%. In a insert-type continuous casting nozzle comprising a highly functional layer formed to have a high corrosion resistance, a high anti-attachment capability, etc., and provided to define an inner bore thereof, the present invention is directed to providing a refractory material (mortar) for an intermediate layer of the continuous casting nozzle, which has a property capable of fixing an inner bore-side layer to an outer periphery-side layer (a nozzle body) of the continuous casting nozzle, while preventing the occurrence of expansion splitting in the outer periphery-side layer due to a difference in thermal expansion between the inner bore-side and outer periphery-side layers, and a continuous casting nozzle using the refractory material for the intermediate layer. The refractory material for the intermediate layer contains a hollow refractory aggregate in an amount of 10 to 75 volume%, wherein a ratio of an average radius R of each particle of the aggregate to an average wall thickness t of the particle satisfies the following relation: R/t > 10. This refractory material is disposed between an inner bore-side layer (2) and an outer periphery-side layer (3,4) of a continuous casting nozzle.