DESCRIPTION DISC MASTER, DISC MASTER MANUFACTURING METHOD, STAMPER, DISC SUBSTRATE, OPTICAL DISC, AND OPTICAL DISC MANUFACTURING METHOD Technical Field [0001] The present invention relates to a disc master, a disc master manufacturing method, a stamper, a disc substrate, an optical disc, and an optical disc manufacturing method. Background Art [0002] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-152465 Patent Document 2: Japanese Unexamined Patent Application Publication No. 2-150325 [0003] Conventionally, in a general optical disc manufacturing method, first, a disc master is manufactured. Then, a stamper is manufactured using the disc master, and disc substrates are mass-produced using the stamper. A layered structure including a reflective film, a cover layer, etc., is formed on the mass-produced disc substrates. Thus, optical discs are completed. Here, first, the disc master is formed such that the disc master has a recess-projection pattern for forming a pit/land structure which defines information signal lines, or a recess-projection pattern for forming a groove/land structure which defines recording tracks. Then, a stamper is formed in which the recess-projection pattern is transferred in an inverted manner. Then, disc substrates having a recess-projection pattern obtained by transferring the recess-projection pattern on the stamper in an inverted manner are manufactured. [0004] A part of a manufacturing process for manufacturing, for example, a reproduction-only optical disc having embossed pit rows defined by a pit/land structure will be described with reference to Figs. 8 and 9. Fig. 8(a) illustrates the manner in which a photoresist (organic resist) film formed on a master substrate made of, for example, glass, is subjected to an exposure step. In the process of forming a disc master, a resist film 102 is formed on the master substrate, and the resist film 102 is irradiated with a laser beam L. The laser beam L is modulated on the basis of information signals to be recorded in the form of pit rows. As shown in Fig. 8(a), portions of the resist film 102 which are subjected to the laser irradiation are formed into exposed portions 102a by optical reaction. In other words, the exposed portions 102a and unexposed portions 102b are formed as a result of the laser irradiation. [0005] After the above-described exposure step, development is performed in a development step. As a result, as shown in Fig. 8 (b), the exposed portions 102a are formed into recessed portions 110 and the unexposed portions 102b are formed into projecting portions 111. Thus, a disc master having a physical recess-projection pattern is completed. In other words, the disc master is manufactured by subjecting the organic resist to exposure with optical reaction and then developing the organic resist. Here, in the disc master obtained by the exposure with optical reaction, boundaries between the recessed and projecting portions are generally substantially vertical. In Figs. 8(a) and 8(b), the boundaries between the recessed and projecting portions are inclined. The boundaries having the shapes shown in the figures can be formed by the following method. That is, when the development of the disc master is performed, diffracted light (0 order light and 1st order light) is observed on a development monitor and the development is stopped in the state in which an optimum signal can be obtained instead of performing the development to the end. In this case, a higher separation performance can be obtained compared to the case in which the boundaries are closer to vertical. The development using the development monitor is commonly performed in the optical reaction method. [0006] Next, a stamper 104 shown in Fig. 8(c) is manufactured using the above-described disc master. The stamper 104 has a recess-projection pattern obtained by transferring the recess-projection pattern on the disc master in an inverted manner. More specifically, portions corresponding to the recessed portions 110 in the disc master are formed as projecting portions 120 and portions corresponding to the projecting portions 111 in the disc master are formed as recessed portions 121. [0007] Disc substrates are mass-produced by injection molding using the above-described stamper 104. Fig. 9(a) shows the state in which resin (for example, polycarbonate) for forming a disc substrate 105 is injected into a mold in which the stamper 104 is placed. Fig. 9(b) shows the state in which the resin injected into the mold is cooled. Fig. 9(c) shows the state in which the stamper 104 is removed after the resin is cooled. Thus, as shown in the figure, the disc substrate 105 in which the recess- projection pattern on the stamper 104 is transferred in an inverted manner is obtained. In the disc substrate 105, portions corresponding to the recessed portions 121 in the stamper are formed as projecting portions (lands) 131 and portions corresponding to the projecting portions 120 in the stamper are formed as recessed portions (pits) 130. Disclosure of Invention [0008] In the disc substrate 105 produced by injection molding using the stamper 104 as described above, when the recess-projection pattern is formed, portions with a small radius of curvature are formed in regions (hereinafter referred to as "recess-projection boundaries") where the shape changes from the recessed portions 130 to the projecting portions 131 or from the projecting portions 131 to the recessed portions 130. In these regions, high residual stress remains. Therefore, to ensure the shape-maintaining characteristics (hereinafter referred to as "transferability") of the pits in the disc substrate 105 after,the disc substrate 105 is separated from the stamper 104, sufficient cooling and pressurization must be performed to release the residual stress. However, when sufficient cooling and pressurization are performed to improve the transferability, there is a problem that adhesion between the substrate and the stamper increases and the separation performance will be degraded. [0009] Various methods for improving the separation performance by, for example, selecting a material which can be easily separated from the stamper 104 as a material of the disc substrate 105, changing the molding conditions of the disc substrate 105, changing the shape of the mold for holding the stamper 104, subjecting the stamper 104 to a surface treatment for improving the separation performance, mixing a releasing agent into the material for forming the substrate, or changing the overall shape of the stamper 104, have been proposed and tested. However, these methods are disadvantageous in that signal characteristics or physical characteristics will be degraded or in that the manufacturing steps are complex, and the stable separation performance of the stamper 104 cannot be obtained. [0010] As another method, the shapes of portions with a small radius of curvature (for example, edges of pits) at the recess-projection boundaries have been changed into a softer shape to increase the radius of curvature. For example, in Fig. 9(b), the radius of curvature is increased by utilizing the fact that the edges at the recess-projection boundaries are rounded due to curing contraction of the material for forming the disc substrate 105 when the material is pressurized and cooled. As shown in Fig. 9(c), regions in which the projecting portions 131 continue to the recessed portions 130 are formed in a curved shape. In this case, the effect of reduction in the contact area between the stamper 104 and the disc substrate 105 is additionally obtained and stable separation performance can be provided. However, the difference at the recess-projection boundaries, that is, pit/land boundaries, in the disc substrate 105 becomes unclear. This leads to another problem that signal characteristics will be degraded when signals are read from the optical disc. [0011] Fig. 10 is an enlarged view illustrating the recess-projection pattern on the disc substrate 105 obtained when the radius of curvature is increased. In the disc substrate 105, the recessed portions 130 serve as pits and the projecting portions (non-recessed portions) 131 serve as lands. As shown in the figure, recess-projection boundaries 132 between the recessed portions 130 and the projecting portions 131 have a curved shape with a relatively large radius of curvature. Here, the ideal shape of the contour of the recess- projection boundaries 132 in consideration of the reproduction signal characteristics is the shape shown by the dashed line (M). However, since the recess-projection boundaries 132 are curved, there are areas A and B where the actual contour differs from the ideal contour M. In the area A, the actual contour differs from the ideal contour of the land portion. In the area B, the actual contour differs from the ideal inclined pit portion. [0012] In this case, in a portion corresponding to the area A, the position of the pit/land boundary in the area A is unclear. Therefore, when the optical disc is completed and is subjected to a reproduction process, the pit/land boundary cannot be clearly detected in the reproduction signal. As a result, degradation of the signal characteristics, such as increase of jitter in the reproduction signal, will occur. [0013] In view of the above-described problems, an object of the present invention is .to ensure the separation performance while maintaining the maintenance of the signal characteristics (suitable transferability) of the optical disc. [0014] The present invention provides a disc master, a stamper, and a disc substrate used in a manufacturing process for manufacturing an optical disc, and the optical disc as a completed product. The manufacturing process includes the steps of manufacturing the stamper using the disc master, the disc master having a recessed portion formed therein, the stamper having a projecting portion formed by transferring the recessed portion; manufacturing the disc substrate using the stamper, the disc substrate having a recessed portion formed by transferring the projecting portion in the stamper; and forming a predetermined layered structure on the disc substrate. In addition, the present invention also provides a disc master manufacturing method and an optical disc manufacturing method which relate to the above-mentioned manufacturing process. [0015] In the disc master according to the present invention, an exposed portion is formed by thermochemical reaction caused by exposing an inorganic resist film provided on a master substrate to a laser beam, a development process is performed so that the exposed portion is formed into the. recessed portion, and a boundary portion between the recessed portion and a non-recessed portion includes a small protrusion which protrudes from a flat surface of the non-recessed portion. In addition, a height of the small protrusion from the flat surface of the non-recessed portion is in the range of 3% to 10% of a height of the flat surface of the non-recessed portion from the recessed portion, and a radius of curvature of the small protrusion is in the range of 20% to 60% of the height of the flat surface of the non-recessed portion from the recessed portion. [0016] The disc master manufacturing method according to the present invention includes a film forming step of forming an inorganic resist film on a master substrate; an exposure step of forming an exposed portion by thermochemical reaction by exposing the inorganic resist film on the master substrate to a laser beam; and a deposition step of performing a deposition process for the master substrate on which the exposed portion is formed for a predetermined time, so that the exposed portion is formed into the recessed portion and a small protrusion which protrudes from a flat surface of a non-recessed portion is formed in a boundary portion between the recessed portion and the non-recessed portion. In this case, an inorganic resist material for forming the inorganic resist film, power of the laser beam, and the time for which the development process is performed are set such that a height of the small protrusion from the flat surface of the non-recessed portion is in the range of 3% to 10% of a height of the flat surface of the non-recessed portion from the recessed portion and a radius of curvature of the small protrusion is in the range of 20% to 60% of the height of the flat surface of the non-recessed portion from the recessed portion. [0017] In the stamper according to the present invention, a boundary portion between the projecting portion and a non-projecting portion includes a small depression which is formed in a bottom surface of the non-projecting portion. [0018] In the disc substrate according to the present invention, a boundary portion between the recessed portion and a non-recessed portion includes a small protrusion which protrudes from a flat surface of the non-recessed portion. In this case, a height of the small protrusion from the flat surface of the non-recessed portion is in the range of 3% to 10% of a height of the flat surface of the non-recessed portion from the recessed portion, and a radius of curvature of the small protrusion is in the range of 20% to 60% of the height of the flat surface of the non-recessed portion from the recessed portion. [0019] In the optical disc according to the present invention, a boundary portion between the recessed portion and a non- recessed portion formed in the disc substrate includes a small protrusion which protrudes from a flat surface of the non-recessed portion. In addition, a height of the small protrusion from the flat surface of the non-recessed portion is in the range of 3% to 10% of a height of the flat surface of the non-recessed portion from the recessed portion, and a radius of curvature of the small protrusion is in the range of 20% to 60% of the height of the flat surface of the non-recessed portion from the recessed portion. [0020] The optical disc manufacturing method according to the present invention includes a film forming step of forming an inorganic resist film on a master substrate; an exposure step of forming an exposed portion by thermochemical reaction by exposing the inorganic resist film on the master substrate to a laser beam; a deposition step of forming a disc master by performing a deposition process for the master substrate on which the exposed portion is formed for a.predetermined time so that the exposed portion is formed into the recessed portion and a small protrusion which protrudes from a flat surface of a non-recessed portion is formed in a boundary portion between the recessed portion and the non-recessed portion; a stamper forming step of forming a stamper using the disc master, the stamper having a projecting portion formed by transferring the recessed portion in the disc master; a substrate forming step of forming a disc substrate using the stamper, the disc substrate having a recessed portion formed by transferring the projecting portion in the stamper; and a layered-structure forming step of forming an optical disc by forming a predetermined layered structure on the disc. [0021] In addition, in the disc substrate according to the present invention, the disc substrate has a recessed portion formed by transferring the projecting portion in the stamper, and the stamper used for manufacturing the disc substrate is a stamper in which a boundary portion between the projecting portion and a non-projecting portion includes a small depression which is formed in a bottom surface of the non-projecting portion. In addition, in the optical disc according to the present invention, the stamper used for manufacturing the disc substrate is a stamper in which a boundary portion between the projecting portion and a non-projecting portion includes a small depression which is formed in a bottom surface of the non-projecting portion, and the predetermined layered structure is formed on the disc substrate. [0022] According to the above-described present invention, in a process of manufacturing a disc master, a stamper, a disc substrate, and an optical disc, separation performance between the stamper and the disc substrate and maintenance of satisfactory signal characteristics after the optical disc is manufactured are taken into consideration in the step of manufacturing the disc master. More specifically, in the disc master, a boundary portion between a recessed portion and a non-recessed portion includes a small protrusion which protrudes from a flat surface of the non-recessed portion. The shape of the disc master is inversely transferred to the stamper, and the shape of the stamper is inversely transferred to the disc substrate. The separation performance is improved and the signal characteristics are maintained due to the shape of the boundary portion. [0023] According to the present invention, in the process of manufacturing the optical disc, the separation performance between the stamper and the disc substrate can be improved. In addition, satisfactory signal characteristics can be maintained in the manufactured optical disc. Brief Description of Drawings [0024] [Fig. 1] Fig. 1 is a diagram illustrating manufacturing steps according to an embodiment of the present invention. [Fig. 2] Fig. 2 is a diagram illustrating manufacturing steps according to the embodiment. [Fig. 3] Fig. 3 is a diagram illustrating PTM. [Fig. 4] Fig. 4 is a diagram illustrating small protrusions on a disc master and small depressions in a stamper according to the embodiment. [Fig. 5] Fig. 5 is a diagram illustrating the small protrusions on the disc substrate according to the embodiment. [Fig. 6] Fig. 6 is a diagram illustrating a pit/land pattern including the small protrusions on the disc substrate according to the embodiment. [Fig. 7] Fig. 7 shows an AFM photograph of the pit/land pattern according to the embodiment and a diagram illustrating the cross-sectional shape of the pit/land pattern. [Fig. 8] Fig. 8 is a diagram illustrating manufacturing steps in the case of optical reaction. [Fig. 9] Fig. 9 is a diagram illustrating manufacturing steps in the case of pptical reaction. [Fig. 10] Fig. 10 is a diagram illustrating the pit/land pattern in the case of optical reaction. Best Mode for Carrying Out the Invention [0025] Hereinafter, an embodiment of the present invention will be described. First, the overall manufacturing process of an optical disc will be described with reference to Figs. I and 2. Fig. 1(a) shows the state in which a resist film 2 is formed on a master substrate 1 for forming a disc master. The master substrate 1 is, for example, a glass substrate or a silicon wafer substrate. In the film forming step, the resist layer 2 made of an inorganic resist material is uniformly formed on the master substrate 1 by sputtering. More specifically, a film of inorganic resist is formed using a film-forming device (sputtering device) on the master substrate 1 made of glass or silicon wafer. Thus, a film with a sufficient thickness for forming pits or grooves with a desired height is obtained. In the sputtering device, an alloy oxide of a transition metal, for example, is used as a target material. With regard to a film-forming method, DC or RF sputtering is used. In this example, in a mastering process for forming the disc master, PTM mastering using an inorganic resist material is performed. In this case, an incomplete oxide of a transition metal is used as a material for forming the resist layer 2. Examples of transition metals, which will be described below, include Ti, V, Cr, Mn, Fe, Nb, Cu, Ni, Co, Mo, Ta, W, Zr, Ru, and Ag. [0026] Next, in an exposure step, as shown in Fig. l(b), an exposure laser beam L is emitted using a mastering device and the resist layer 2 is selectively irradiated with the exposure laser beam L in accordance with pit rows or grooves corresponding to a signal pattern. In this case, recording information is prepared in advance and is stored in a signal transmitter (formatter). Then, the laser beam L is modulated by a signal output from the signal transmitter, so that the exposure process can be performed in accordance with, for example, the pit rows. In the case where, for example, a Blu-ray Disc (Blu-ray Disc: registered trademark) is manufactured, a blue laser diode with a wavelength of 405 nm is used as a source of the exposure laser beam L. In the exposure step, portions irradiated with the exposure laser beam L are formed into exposed portions 2a by thermochemical reaction. Thus, the exposed portions 2a and unexposed.portions 2b are formed. [0027] Next, in a development step, the resist layer 2 is developed (etched) so that a disc master 3 having a predetermined recess-projection pattern (pit rows or grooves) is produced. In this case, after the above-described exposure step, the development step is performed by a development device using alkaline fluid to obtain the disc master. A developing method may be, for example, a dipping method including an immersing step or a method in which a chemical is applied to a base while the base is rotated by a spinner. With regard to the developer, for example, an organic alkaline developer containing TMAH (tetramethylammonium hydroxide) as a main component or an inorganic alkaline developer, such as KOH, NaOH, a phosphoric-acid-based developer, etc., may be used. In the disc master 3, as shown in Fig. l(c), the exposed portions 2a are formed into recessed portions 10 and the unexposed portions 102b are formed into projecting portions (non-recessed portions) 11. Thus, the disc master 3 having a physical recess-projection pattern is completed. In other words, the disc master 3 is manufactured by subjecting the inorganic resist to exposure with thermochemical reaction and then developing the inorganic resist. [0028] Next, in an electroforming step, a stamper 4 is manufactured using the above-described disc master 3. After the above-described development step, the disc master 3 is washed with water. Then, a metal nickel film 4a is deposited on a surface of the disc master 3 having the recess-projection pattern in an electroforming bath, as shown in Fig. 1(d). After the electroforming step, the master obtained by the development step and the metal master are separated from each other (Fig. l(e)). Then, after the disc master 3 is removed, a predetermined process is performed so that the stamper 4 in which the recess-projection pattern on the disc master 3 is transferred and which is used for forming substrates is obtained. The stamper 4 has a recess-projection pattern obtained by transferring the recess-projection pattern on the disc master 3 in an inverted manner. More specifically, portions corresponding to the recessed portions 10 in the disc master 3 are formed as projecting portions 20 and portions corresponding to the projecting portions 11 in the disc master 3 are formed as recessed portions (non-projecting portions) 21. [0029] Here, before the electroforming step, the surface of the master obtained by the development step may be subjected to a separation treatment to improve the separation performance. This process is performed as necessary. Then, after the nickel stamper 4 is manufactured, the disc master 3 made of inorganic resist is stored after being washed with water and dried. A desired number of nickel stampers are repeatedly manufactured as necessary. In addition, as necessary, the stamper 4 separated from the master obtained by the development step may be used as a master for forming a mother master having the same recess-projection pattern as that on the master obtained by the development step by newly performing the electroforming step and the separation step. Then, the thus-obtained mother master may be used as a new disc master 3 for manufacturing another stamper having the same recess-projection pattern as that on the stamper 4 by newly performing the electroforming step and the separation step. [0030] The stamper 4 (Fig. 2(a)) manufactured by the steps shown in Figs. l(a) to l(e) is used to form a resin disc substrate 5 made of, for example, polycarbonate, which is a thermoplastic resin, by injection molding or the like. More specifically, resin is injected into a mold (not shown) while the stamper 4 is placed in the mold, and is then cured. Then, th'e resin is separated from the stamper 4 Thus, the disc substrate 5 is formed (Figs. 2(b) and 2(c)). As shown in Fig. 2(c), the thus-formed disc substrate 5 has a recess-projection pattern in which portions corresponding to the projecting portions 20 in the stamper 4 are formed as recessed portions 30 and portions corresponding to the recessed portions (non-projecting portions) 21 in the stamper 4 are formed as projecting portions 31. The recessed portions 30 serve as pits and projecting portions (non-recessed portions) 31 serve as lands. [0031] Then, as shown in Fig. 2(d), the disc substrate 5 is subjected to a process for forming a reflective film 6 made of, for example, an Ag alloy on the surface of the disc substrate 5 having the recess-projection pattern by sputtering. Then, as shown in Fig. 2(e), a cover layer 7 is formed. The cover layer (light transmitting layer) 7 is formed by spreading, for example, ultraviolet curable resin by spin coating and then curing the resin by irradiating the resin with ultraviolet rays. In the state shown in Fig. 2(e), the optical disc is completed. Additionally, however, the surface of the cover layer 7 may be subjected to hard coating and a moisture barrier film, may be formed on the surface at the opposite side. [0032] In the above-described manufacturing process, the resist material of the resist layer 2 used for manufacturing the disc master 3 is an incomplete oxide of a transition metal, as described above. Here, an incomplete oxide of a transition metal is defined as a compound in which the oxygen content is smaller than that in the stoichiometric composition which corresponds to the valency of the transition metal, that is, a compound in which the content of oxygen in the incomplete oxide of the transition metal is smaller than the oxygen content in the stoichiometric composition which corresponds to the valency of the transition metal. For example, a chemical formula MoO3 will be described as an example of an oxide of a transition metal. When the composition in the oxidation state of the chemical formula Mo03 is expressed as Mo1_xOx, a complete oxide is obtained when x=0.75. In contrast, when x is in the range of 0 An organic alkaline developer containing TMAH (tetramethylammonium hydroxide) as a main component, which is generally used with semiconductors. An inorganic resist containing an incomplete oxide of a transition metal as a main component. (the transition metal is Ti, V, Cr, Mn, Fe, Nb, Cu, Ni, Co, Mo, Ta, W, Zr, Ru, Ag, etc.) A blue-violet laser beam with a wavelength of 405 nm emitted at an output level of 10 to 15 mW and a linear velocity of about 4.9 m/s (it is assumed that a Blu-ray disc is manufactured and the capacity of the Blu-ray disc is 25 GB (Giga Byte) for each layer). Development is performed about 10 minutes using the above-mentioned developer. [0054] The disc master 3 was manufactured under the above-described conditions. The height and the radius of curvature of the small protrusions 32 were as described below. That is, the observed value of the height of the protrusions was 3 to 5 nm. The rate of the height of the protrusions with respect to the vertical distance (recess-projection vertical interval) between the recessed portions 10 (pit portions) and the non-recessed portions 11 (land portions), which is 70 nm in the case of the Blu-ray disc, is 3/70 to 5/70. Thus, the height of the protrusions is 4% to 7% of the pit/land vertical interval. In the case of the DVD (Digital Versatile Disc), the recess-projection vertical interval is 120 to 130 nm. Therefore, the rate of the height of the protrusions is 3/130 to 5/120 (2% to 4% with respect to the vertical interval). [0055] The observed value of the radius of curvature was 20 to 40 nm. In the case of the Blu-ray disc, the rate of the radius of curvature with respect to the recess-projection vertical interval, which is 70 nm, is 20/70 to 40/70 (29% to 57% with respect to the vertical interval), that is, 30% to 60%. In the case of the DVD, the rate of the radius of curvature with respect to the recess-projection vertical interval, which is 120 to 130 nm, is 20/130 to 40/120 (15% to 33% with respect to the vertical interval), that is, 15% to 35%. [0056] The height and the radius of curvature of the small protrusions 12 affect the depth and the radius of curvature of the small depressions 23 in the stamper 4. Then, the depth and the radius of curvature of the small protrusions 32 on the disc substrate 5 are substantially similarly affected. In the case of the Blu-ray disc, the height of the small protrusions 32 shown in Fig. 6 is 4% to 7% of the pit/land height, and the radius of curvature thereof is 30% to 60% of the pit/land height. Thus, in these cases, the contraction of the substrate due to cooling of the resin in the molding process can be compensated for, and the cooling time can be reduced. Thus, both the transferability and the separation performance can be ensured. [0057] Here, in the above embodiment, a case in which a reproduction-only disc is manufactured and the recess-projection pattern corresponds to pits and lands has been described. However, the present invention can also be applied to the case in which a recordable disc is manufactured, that is, to the case in which the recess-projection pattern corresponds to grooves and lands, or to the case in which a recess-projection transferring process for multi-layered holograms is performed. In addition, the present invention can also be applied in the process of forming fine shapes on a surface. CLAIMS 1. A disc master for use in a manufacturing process for manufacturing an optical disc, the manufacturing process including the steps of manufacturing a stamper using the disc master, the disc master having a recessed portion formed therein, the stamper having a projecting portion formed by transferring the recessed portion; manufacturing a disc substrate using the stamper, the disc substrate having a recessed portion formed by transferring the projecting portion in the stamper; and forming a predetermined layered structure on the disc substrate, wherein the disc master is characterized in that an exposed portion is formed by thermochemical reaction caused by exposing an inorganic resist film provided on a master substrate to a laser beam, a development process is performed so that the exposed portion is formed into the recessed portion, and a boundary portion between the recessed portion and a non-recessed portion includes a small protrusion which protrudes from a flat surface of the non-recessed portion. 2. The disc master according to claim 1, characterized in. that a height of the small protrusion from the flat surface of the non-recessed portion is in the range of 3% to 10% of a height of the flat surface of the non-recessed portion from the recessed portion, and a radius of curvature of the small protrusion is in the range of 20% to 60% of the height of the flat surface of the non-recessed portion from the recessed portion. 3. A disc master manufacturing method for manufacturing a disc master for use in a manufacturing process for manufacturing an optical disc, the manufacturing process including the steps of manufacturing a stamper using the disc master, the disc master having a recessed portion formed therein, the stamper having a projecting portion formed by transferring the recessed portion; manufacturing a disc substrate using the stamper, the disc substrate having a recessed portion formed by transferring the projecting portion in the stamper; and forming a predetermined layered structure on the disc substrate, wherein the disc master manufacturing method is characterized by comprising: a film forming step of forming an inorganic resist film on a master substrate; an exposure step of forming an exposed portion by thermochemical reaction by exposing the inorganic resist film on the master substrate to a laser beam; and a deposition step of performing a deposition process for the master substrate on which the exposed portion is formed for a predetermined time, so that the exposed portion is formed into the recessed portion and a small protrusion which protrudes from a flat surface of a non-recessed portion is formed in a boundary portion between the recessed portion and the non-recessed portion. 4. The disc master manufacturing method according to claim 3, characterized in that an inorganic resist material for forming the inorganic resist film, power of the laser beam, and the time for which the development process is performed are set such that a height of the small protrusion from the flat surface of the non-recessed portion is in the range of 3% to 10% of a height of the flat surface of the non- recessed portion from the recessed portion and a radius of curvature of the small protrusion is in the range of 20% to 60% of the height of the flat surface of the non-recessed portion from the recessed portion. 5. A stamper for use in a manufacturing process for manufacturing an optical disc, the manufacturing process including the steps of manufacturing the stamper using a disc master having a recessed portion formed therein, the stamper having a projecting portion formed by transferring the recessed portion; manufacturing a.disc substrate using the stamper, the disc substrate having a recessed portion formed by transferring the projecting portion in the stamper; and forming a predetermined layered structure on the disc substrate, wherein the stamper is characterized in that a boundary portion between the projecting portion and a non-projecting portion includes a small depression which is formed in a bottom surface of the non-projecting portion. 6. A disc substrate manufactured using a stamper, the stamper being manufactured using a disc master having a recessed portion formed therein, the stamper having a projecting portion formed by transferring the recessed portion, the disc substrate having a recessed portion formed by transferring the projecting portion in the stamper, wherein the disc substrate is characterized in that a boundary portion between the recessed portion and a non-recessed portion includes a small protrusion which protrudes from a flat surface of the non-recessed portion. 7. The disc substrate according to claim 6, characterized in that a height of the small protrusion from the flat surface of the non-recessed portion is in the range of 3% to 10% of a height of the flat surface of the non-recessed portion from the recessed portion, and a radius of curvature of the small protrusion is in the range of 20% to 60% of the height of the flat surface of the non-recessed portion from the recessed portion. 8. An optical disc manufactured by the steps of manufacturing a stamper using a disc master having a recessed portion formed therein, the stamper having a projecting portion formed by transferring the recessed portion; manufacturing a disc substrate using the stamper, the disc substrate having a recessed portion formed by transferring the projecting portion in the stamper; and forming a predetermined layered structure on the disc substrate, wherein the optical disc is characterized in that a boundary portion between the recessed portion and a non-recessed portion formed in the disc substrate includes a small protrusion which protrudes from a flat surface of the non-recessed portion. 9. The optical disc according to claim 8, characterized in that a height of the small protrusion from the flat surface of the non-recessed portion is in the range of 3% to 10% of a height of the flat surface of the non-recessed portion from the recessed portion, and a radius of curvature of the small protrusion is in the range of 20% to 60% of the height of the flat surface of the non-recessed portion from the recessed portion. 10. An optical disc manufacturing method, characterized by comprising: a film forming step of forming an inorganic resist film on a master substrate; an exposure step of forming an exposed portion by thermochemical reaction by exposing the inorganic resist film on the master substrate to a laser beam; a deposition step of forming a disc master by performing a deposition process for the master substrate on which the exposed portion is formed for a predetermined time so that the exposed portion is formed into the recessed portion and a small protrusion which protrudes from a flat surface of a non-recessed portion is formed in a boundary portion between the recessed portion and the non-recessed portion; a stamper forming step of forming a stamper using the disc master, the stamper having a projecting portion formed by transferring the recessed portion in the disc master; a substrate forming step of forming a disc substrate using the stamper, the disc substrate having a recessed portion formed by transferring the projecting portion in the stamper; and a layered-structure forming step of forming an optical disc by forming a predetermined layered structure on the disc. 11. A disc substrate manufactured using a stamper, the stamper being manufactured using a disc master having a recessed portion formed therein, the stamper having a projecting portion formed by transferring the recessed portion, the disc substrate having a recessed portion formed by transferring the projecting portion in the stamper, wherein the disc substrate is characterized in that the stamper used for manufacturing the disc substrate is a stamper in which a boundary portion between the projecting portion and a non-projecting portion includes a small depression which is formed in a bottom surface of the non-projecting portion. 12. An optical disc manufactured by the steps of manufacturing a stamper using a disc master having a recessed portion formed therein, the stamper having a projecting portion formed by transferring the recessed portion; manufacturing a disc substrate using the stamper, the disc substrate having a recessed portion formed by transferring the projecting portion in the stamper; and forming a predetermined layered structure on the disc substrate, wherein the optical disc is characterized in that the stamper used for manufacturing the disc substrate is a stamper in which a boundary portion between the projecting portion and a non-projecting portion includes a small depression which is formed in a bottom surface of the non-projecting portion, and the predetermined layered structure is formed on the disc substrate.