FORM 2 THE PATENT ACT 197 0 (39 of 1970) & The Patents Rules, 2003 COMPLETE SPECIFICATION (See Section 10, and rule 13) 1. TITLE OF INVENTION METHOD FOR MANUFACTURING VALVE UMBRELLA PORTION OF HOLLOW ENGINE VALVE, PRESS DEVICE OF VALVE UMBRELLA PORTION OF HOLLOW ENGINE VALVE, AND HOLLOW ENGINE VALVE 2. APPLICANT(S) a) Name : MITSUBISHI HEAVY INDUSTRIES, LTD. b) Nationality : JAPANESE Company c) Address : 16-5, KONAN 2-CHOME, MINATO-KU, TOKYO 1088215, JAPAN AND a) Name : YOSHIMURA COMPANY b) Nationality : JAPANESE Company c) Address : 2685-173, Aza Minamihara, Oaza Nakashidami, Moriyama-ku, Nagoya-shi, Aichi 4630002, JAPAN 3. PREAMBLE TO THE DESCRIPTION The following specification particularly describes the invention and the manner in which it is to be performed : - Technical Field This invention relates to a method for manufacturing a valve umbrella portion of a hollow engine valve which has a valve umbrella portion hollow open at a side to be welded to a shaft end sealing material or to a hollow shaft portion, and in which the valve umbrella portion hollow is formed with an expanded diameter within an expanded-diameter section of the valve umbrella portion, and the maximum inner diameter of the valve umbrella portion hollow is larger than the maximum outer diameter of the hollow shaft portion; a press device for the valve umbrella portion of the hollow engine valve; and the hollow engine valve having the valve umbrella portion. Background Art In regard to a method for manufacturing a valve umbrella portion of a hollow engine valve, the inventor of the present application made the invention of Patent Document 1 to be described below. Its outline will be given as follows: The valve umbrella portion of the hollow engine valve, particularly in an exhaust valve, is exposed to high temperatures. For the valve umbrella portion, therefore, use has been made of materials showing excellent properties including heat resistance, such as heat resisting steels based on manganese, nickel, chromium, etc. These materials have the advantage of being highly resistant to heat, but also has the disadvantage of being poor in plastic workability. That is, it is difficult to forge them into the finished form of the valve umbrella portion, and a hollow has to be provided in the hollow engine valve, thus making working even more difficult. Thus, in forming the valve umbrella portion by forging such a material, it has been common practice to raise the temperature of the material to a value equal to or higher than its recrystallization temperature, and carry out working by hot forging. With hot forging, however, the results are necessarily poor such that working accuracy declines owing to the problem of metal expansion or the like, and that the texture of the surface of the product is inferior to that in cold forging. Under these circumstances, the inventor of the present application sought a method for forming a valve umbrella portion by cold forging, not hot forging, with the use of a material with high heat resistance as mentioned above. By trial and error, the inventor worked out a method comprising producing, first, a valve umbrella portion semifinished product in which the maximum outer diameter of an expanded-diameter section agrees with the maximum outer diameter of the valve umbrella portion as a finished product, and which has a cylindrical hollow with a bottomed lower end having the same inner diameter as the maximum inner diameter of a valve umbrella portion hollow of the valve umbrella portion as the finished product; and then gradually drawing the semifinished product in a plurality of stages by cold forging, with the upper part of the expanded-diameter section and the body being targeted, to prepare the valve umbrella portion as the finished product. The inventor filed an application for this method, and this application was granted a patent right (Patent Document 1 to be described below). As the materials for the valve umbrella portion, the following three types were named: NCF 47W (nickel-based steel) SUH 35 (austenitic manganese-based steel) Inconel 751 (nickel-based steel) The inventor of the present application repeated, many times, confirmation experiments on the above materials even after acquisition of the patent right of the Patent Document 1. As a result, NCF 47W and Inconel 751 were confirmed to obtain a valve umbrella portion as a finished product, without any problem, by the method of the Patent Document 1. Materials with a high carbon content included in JIS 4311 heat resisting steels (SUH 35 also included therein) were found to show trouble, such as cracking or deformation, slightly more frequently than NCF 47W and Inconel 751, when all stages of necking (drawing) were performed by cold forging. In recent years, demand has tended to surge for the low fuel consumption of vehicles, and there has been a tendency to demand that all vehicle components be compact and lightweight. With such trends, hollow engine valves have attracted attention, particularly, because of the weight reduction of members which repeat rapid reciprocating motions within an engine. The tendency is growing toward a keen desire for highly accurate forging using various materials including materials with unsatisfactory cold forgeability. Prior Art Documents Patent Documents Patent Document 1: Japanese Patent No. 4,390,291 Non-Patent Documents Non-patent document 1: "Heat treatment of steel, revised 5th edition" edited by The Iron and Steel Institute of Japan, published by Maruzen Co., Ltd. in 1979 (2nd Ed., 3rd Issue) Non-patent document 2: "The Physical Metallurgy of Steels", W.C. Leslie, supervising translation by N. Koda, published by Maruzen Co., Ltd. in 1987 (2nd Issue) Summary of the Invention Problems to be solved by the invention Various steel materials are conceivable as materials for hollow engine valves, but there are a few of them with cold forgeability. Assume, for example, that a valve umbrella portion is to be formed by cold forging using a material with a high carbon content included in JIS 4311 heat resisting steels. In this case, in order to keep down the incidence rate of defective products, it is necessary to increase the number of steps for drawing, that is, to increase the number of dies. Alternatively, cold forging has to be performed, with an intermediate heat treatment process (such as annealing) being interposed many times between the steps. Anyway, time and labor increase, necessarily rebounding on product prices. Therefore, the development of a method capable of shaping using a material with a high carbon content included in JIS 4311 heat resisting steels, or using other material with poor cold forgeability, without increasing the number of steps, and while minimizing the execution of intermediate heat treatment, has become an impending challenge for the present application to tackle. Means for solving the problems The present invention has been accomplished in an attempt to solve the above-described problems, and provides the following means for solving the problems (solution means): A method for manufacturing a valve umbrella portion of a hollow engine valve having a valve umbrella portion hollow which opens at a side to be welded to a hollow shaft portion or to a shaft end sealing material, the valve umbrella portion hollow being formed with an expanded diameter within an expanded-diameter section of the valve umbrella portion, the maximum inner diameter of the valve umbrella portion hollow being larger than the maximum outer diameter of the hollow shaft portion the method for manufacturing, comprising: a first step of producing the valve umbrella portion semifinished product from a round solid bar as a raw material; and a second step of converting the valve umbrella portion semifinished product into the valve umbrella portion in finished form by hot forging, wherein the first step is adapted to obtain the valve umbrella portion semifinished product which has an expanded-diameter section integral with a body of a cylindrical shape at an end of the body, the maximum outer diameter of the expanded-diameter section being equal to the maximum outer diameter of the expanded-diameter section of the valve umbrella portion as a finished product when an expanded-diameter section side of the valve umbrella portion semifinished product is placed below; and has a cylindrical hollow having an inner diameter equal to the maximum inner diameter of the valve umbrella portion hollow of the finished product the cylindrical hollow being open at the upper end thereof and being bottomed within the expanded-diameter section at the lower end thereof, and the second step is adapted to subject the valve umbrella portion semifinished product to forging at a temperature within a range of room temperature to 870°C to draw an upper part of the expanded-diameter section and the body gradually in a plurality of stages in such a manner as to gradually perform drawing by use of dies for pressing the upper part of the expanded-diameter section and the body of the valve umbrella portion semifinished product, a number of the dies being equal to a number of drawing processes, and inner diameters of the dies decreasing little by little as the stages proceed, while holding an entire space itself including works, the dies, and punches at a constant temperature, thereby obtaining the valve umbrella portion as the finished product in which the maximum inner diameter of the valve umbrella portion hollow within the expanded-diameter section is held at the inner diameter of the cylindrical hollow, and the inner diameter of the valve umbrella portion hollow becomes smaller more upwardly. A press device for a valve umbrella portion of a hollow engine valve, which can produce a valve umbrella portion of a hollow engine valve by the method for manufacturing according to the solution means 1, wherein the press device used in the second step has a heat insulating wall embracing the works, fixtures for fixing the works, the dies, and fixtures for fixing the dies, as a whole, and can hold an interior of the heat insulating wall in a constant temperature state by the effect of the heat insulating wall. A hollow engine valve prepared by welding the valve umbrella portion, which has been produced by the method for manufacturing according to the solution means 1 or by the press device according to the solution means 2, to an end of a shaft end sealing material. A hollow engine valve prepared by welding the valve umbrella portion, which has been produced by the method for manufacturing according to the solution means 1 or by the press device according to the solution means 2, to an end of a hollow shaft portion open at both ends, and welding a shaft end sealing material to the other end of the hollow shaft portion. Effects of the invention According to the invention of the solution means 1 of the present invention, the valve umbrella portion semifinished product is subjected to warm forging at room temperature to 870°C, whereby the entire space itself including works, dies, and punches is held at a constant temperature, and the upper part of the expanded-diameter section and the body are gradually drawn in a plurality of stages. Thus, cracks or deformations are drastically decreased. Moreover, without an increase in the number of steps for necking (drawing), or without the necessity of interposing, many times, intermediate heat treatment such as annealing, the valve umbrella portion can be shaped without problems, even with the use of, say, a material with a high carbon content included in JIS 4311 heat resisting steels. The reason why the shaping of the valve umbrella portion takes place smoothly is nothing but the following: The present invention involves two steps in which the material is once made into the valve umbrella portion semifinished product in the first step, and then it is drawn in the second step to be converted into the valve umbrella portion as a finished product. That is, if the first step of producing the valve umbrella portion semifinished product is lacking, the warm forging at room temperature to 870°C does not make it possible to carry out the drawing as the second step smoothly. Thus, hot forging at an even higher temperature has to be employed. According to the invention of the solution means 2 of the present invention, it is disclosed that a press device for drawing to form a valve umbrella portion finished product is configured to have a heat insulating wall embracing works, fixtures for fixing the works, dies, and fixtures for fixing the dies, as a whole, and to be capable of keeping the interior of the heat insulating wall in a constant temperature state by the effect of the heat insulating wall. In warm forging, what is most problematical is the modification of the structure of the work due to its temperature changes during drawing. That is, if the number of steps for drawing is of the order of one or two, the work preheated to a necessary temperature is drawn by a die or punch incorporating a heater, whereby the work can be processed with little influence by the modification of the work. With an increase in the number of steps for drawing, for example, to 3 or 4 or more, however, the temperature of the work lowers every time the work is exposed to the air, even when the dies or punches are heated by the heaters. As a result, the modification (hardening) of the metal structure proceeds. If the work is forcibly drawn under unchanged conditions, therefore, cracking or the like occurs in the work, and a finished product cannot be obtained. Normally, in case drawing is performed in a somewhat large number of steps, it is a frequent practice to interpose, many times, steps for intermediate heat treatment, such as annealing, in the meantime. For the drawing of the valve umbrella portion of the hollow engine valve, around 10 steps, or more steps in some cases, are needed, although one cannot say definitely, because the situation differs depending on the material. Anyway, the number of the steps for intermediate heat treatment increases, and drawing has to be interrupted for each intermediate heat treatment process. Thus, the intermediate process interposing procedure cannot be a realistic manufacturing method. That is, this procedure, if experimental, is acceptable, but is deemed not to have technical contents applicable to line production in an actual plant. Thus, it becomes necessary to continuously perform drawing in around 10 steps, or more steps in some cases, while minimizing the number of steps for intermediate heat treatment, such as annealing, to be interposed. Of vital importance here is to avoid a temperature fall of the work. A constant temperature is achievable if the heater is incorporated in or annexed to the die or punch. For the work for which the heater is not mountable, on the other hand, a temperature fall at the instant of exposure to the air is unavoidable. To solve this problem, the entire space itself including the works, dies and punches needs to be kept at a constant temperature. The invention of the solution means 2 of the present invention discloses technical contents which allow the entire space to be maintained in a constant temperature atmosphere. Because of the technical contents, the constant temperature state in the drawing of the work is held in the ideal form. Consequently, the temperature fall of the work is avoided during drawing of the work, and drawing in the plural steps can be performed smoothly, without the need to carry out intermediate heat treatment many times. According to the invention of the solution means 3 or the solution means 4 of the present invention, there can be obtained a hollow engine valve as a finished product having the valve umbrella portion obtained by the invention of the solution means 1 or the solution means 2 of the present invention. The grounds for the numerical limitation on the temperature range in the second step described in the solution means 1 are as follows: There are various theories on the definition of the temperature range of warm forging, and no established theory has been presented. In the present invention, a temperature range "equal to or lower than the recrystallization temperature of a steel material", which is considered to be the most common as a temperature range for warm forging, is taken as "the temperature range for warm forging". The meaning of "common", as referred to here, is that "can be applied most widely in various steel materials". Thus, it goes without saying that as the material is restricted, this temperature range can be limited to an even narrower range. Based on the above concept, there is no lower limit on the temperature range of "warm forging". In the actual job site, however, it is a rare practice to forge a material while cooling it. Thus, the lower limit of the temperature range is set at "room temperature". The definition of "room temperature" is also considered variously, but in the present invention, "room temperature" is taken to mean 10°C to 30°C according to common knowledge. The lower limit in the actual operation is assumed to be around 20. Next, the upper limit of the temperature range for "warm forging" is set at 870°C based on the description of Fig. 2.16 on page 48 of the aforementioned non-patent document 1. That is, the recrystallization temperature is not a specific temperature, but fluctuates with conditions. Fig. 2.16 on page 48 of the non-patent document 1 states that in the case of soft-iron, there can be a recrystallization temperature of up to 870°C depending on conditions. Thus, this value has been adopted as the upper limit of the temperature range. In connection with the recrystallization temperature of iron, the aforementioned non-patent document 2 describes on page 138 that the recrystallization temperature varies (rises) if an additive element is contained in iron. The heat resisting steel, one of the materials in the present invention, contains nickel (contained in almost all of austenitic heat resisting steels), molybdenum (contained in SUH 38), and chromium (contained in all of heat resisting steels), which act to raise the recrystallization temperature. From the data described on page 138 of the aforementioned nonpatent document 2, it can be expected that the recrystallization temperature is highly likely to be at least 700°C, although it may vary according to the proportion of an alloying element added to iron. In the present invention, the term "valve umbrella portion as a finished product" or the term "valve umbrella portion in finished form" is used. These terms refer to a valve umbrella portion in the following states: 1) The valve umbrella portion has reached a state in which the outer diameter of the expanded-diameter section does not change any more. 2) The valve umbrella portion has reached a state in which the maximum inner diameter of the hollow does not change any more. 3) The valve umbrella portion has reached a state in which the outer diameter of the end of the body agrees with the outer diameter of the shaft end sealing material or the hollow shaft portion. The valve umbrella portion in the above three states is called "valve umbrella portion as a finished product" or "valve umbrella portion in finished form". Thus, one is free, for example, to stamp a surface of the expanded-diameter section of the valve umbrella portion in finished form in a flat state, or form a concavity there by hot forging. Such an act is essentially processing which is applied later to the "valve umbrella portion as a finished product" or the "valve umbrella portion in finished form" in the present invention. Needless to say, any similar processings, which are performed later using the "valve umbrella portion as a finished product" or the "valve umbrella portion in finished form" in the present invention, are all included in the scope of the present invention, if they employ the method of the present invention in working the valve umbrella portion in the aforementioned three states. The method of the present invention also aims to minimize processes for "annealing". Hence, it is only natural, in view of the above gist, that the scope of the present invention does not exclude a method of interposing one or two intermediate annealing steps in the method of the present invention. That is, in a case where, for example, the number of steps in a rotary press device is too large, the second step is divided into a former half and a latter half to decrease the number of the processes by one, and a step of reheating, i.e., annealing, of the material is interposed between the former half and the latter half. This is a matter-of-course request based on the technical contents. All such methods are included, without doubt, in the scope of the present invention. Brief Description of the Drawings Fig. 1 Fig. 1 is a front view of a press device used in the second step in the manufacturing method of Embodiment 1 of the present invention. Fig. 2 Fig. 2 is the front view of the press device used in the second step in the manufacturing method of Embodiment 1 of the present invention, in which a part of a die set is omitted. Fig. 3 Fig. 3 is a longitudinal sectional view of the press device used in the second step in the manufacturing method of Embodiment 1 of the present invention, in which a part of the die set is omitted. Figs. 4(a) to 4(d) Figs. 4(a) to 4(d) are explanation drawings for illustrating the second step in the manufacturing method of Embodiment 1 of the present invention. Figs. 5(a) to 5(d) Figs. 5(a) to 5(d) are explanation drawings for illustrating the second step in the manufacturing method of Embodiment 1 of the present invention. Figs. 6(a), 6(b) Fig. 6(a) is a vertical sectional view of a valve umbrella portion semifinished product obtained in the first step in the manufacturing method of Embodiment 1 of the present invention. Fig. 6(b) is a vertical sectional view of a valve umbrella portion as a finished product obtained in the second step in the manufacturing method of Embodiment 1 of the present invention. Figs. 7(a) to 7(c) Figs. 7(a) to 7(c) are explanation drawings for illustrating a first method for the first step in the manufacturing method of Embodiment 1 of the present invention. Figs. 8(a) to 8(c) Figs. 8(a) to 8(c) are explanation drawings for illustrating a second method for the first step in the manufacturing method of Embodiment 1 of the present invention. Figs. 9(a), 9(b) Fig. 9(a) is a vertical sectional view of an example of a hollow engine valve obtained in the manufacturing method of Embodiment 1 of the present invention. Fig. 9(b) is a vertical sectional view of another example of the hollow engine valve obtained in the manufacturing method of Embodiment 1 of the present invention. Mode for Carrying Out the Invention The best mode for carrying out the present invention will be described in detail below by reference to the accompanying drawings. Embodiment 1 As Embodiment 1 of the present invention, a method for manufacturing a valve umbrella portion 1, and a hollow engine valve V having the valve umbrella portion 1 will be described in detail as follows: The hollow engine valve V is composed of the valve umbrella portion 1, and a shaft end sealing material 3, as shown in Fig. 9a. That is, the hollow engine valve V is of a structure in which the shaft end sealing material 3 is welded to an end of the valve umbrella portion 1, and a hollow S is provided inside the valve umbrella portion 1. In the hollow S, when the hollow engine valve V is used as an exhaust valve, sodium (not shown) is sealed up. When the hollow engine valve V is not used as an exhaust valve, sodium is not sealed up in the hollow S. A hollow engine valve Y shown in Fig. 9b represents an example in which a hollow shaft portion 2 is welded to a valve umbrella portion 1, and a shaft end sealing material 3 is further welded to the hollow shaft portion 2. Inside the hollow engine valve Y, a hollow S is similarly provided. In the hollow S, when the hollow engine valve Y is used as an exhaust valve, sodium (not shown) is sealed up. When the hollow engine valve Y is not used as an exhaust valve, sodium is not sealed up in the hollow S. As the hollow shaft portion 2 of the hollow engine valve Y, an electric welded tube prepared by welding together the ends of a steel sheet rolled up, or a seamless pipe without seams can be used. Any welding method is available when welding the respective members, but friction welding, for example, can be used. Concrete names for the material for the valve umbrella portion 1 are as follows: When the hollow engine valve V or Y of Embodiment 1 is used as an exhaust valve, a material with high resistance to heat, such as NCF 47W or SUH 35 or Inconel 751, is used for the valve umbrella portion 1. A material with the second highest resistance to heat, for example, SUS 304, SUS 430 or SUH 11, is used for the hollow shaft portion 2 (only in Y). For the shaft end sealing material 3, a material with slightly poor resistance to heat, such as SUH 11, may be used. When the engine valve V or Y is not used as an exhaust valve, on the other hand, a material with so high resistance to heat need not be used for any of the valve umbrella portion 1, the hollow shaft portion 2, and the shaft end sealing material 3. The hollow engine valve V or Y obtained by the manufacturing method of Embodiment 1 of the present invention is as described above. The manufacturing method for the valve umbrella portion 1, which serves as the core for Embodiment 1 of the present invention, will be described in detail below. Fig. 6a shows, in a vertical sectional view, the valve umbrella portion 1 in semifinished form (a semifinished product 11) which is obtained in the first step of Embodiment 1 of the present invention. The semifinished product 11 is formed, with a disk-shaped expanded-diameter section 111 and a cylindrical body 112 as an integral unit. A lower end part of the body 112 is continuously connected to the upper end of the expanded-diameter section 111, and a connecting part gently curves, as shown in Fig. 6a. Inside the semifinished product 11, a cylindrical hollow Sll having a bottomed lower end is formed. The upper end of the hollow Sll opens at an upper surface of the body 112, and its lower end is bottomed within the expanded-diameter section 111. In a second step of Embodiment 1 of the present invention, an upper part of the expanded-diameter section 111 and the whole of the body 112 of the semifinished product 11 in Fig. 6a are subjected to drawing (necking) by warm forging to obtain the valve umbrella portion 1 in finished form as shown in Fig. 6b. In Fig. 6b, la denotes an expanded-diameter section, and lb denotes a body. In the valve umbrella portion 1 as a finished product, it is difficult to determine the boundary between the expanded-diameter section la and the body lb. In Fig. 6b, however, the expanded-diameter section la and the body lb are separated at a site of the contour of the sectional view where the curvature of the curve becomes sharp. S1 denotes a cylindrical hollow bottomed at its lower end, the upper end of the hollow S1 opens at an upper surface of the body lb, and the lower end of the hollow S1 is bottomed within the expanded-diameter section la. In Fig. 6a, hll denotes the height of the entire semifinished product 11; hl2, the height of the expanded-diameter section 111; hl3, the height of the body 112; hl4, the height (depth) of the hollow S11; cplO, the outer diameter of the body 112; φl2, the maximum outer diameter of the expanded-diameter section 111; and φll, the inner diameter of the hollow Sll. In Fig. 6b, h15 denotes the height of the entire valve umbrella portion 1 as a finished product; h1 6, the height of the expanded-diameter section la; h17, the height of the body lb; hl8, the height (depth) of the hollow S1; φl4, the outer diameter of an upper end part of the body lb; φl2, the maximum outer diameter of the expanded-diameter section la; cpll, the maximum inner diameter of the hollow S1; and φl3, the inner diameter of an upper end part of the hollow S1. The height hl5 of the entire valve umbrella portion 1 as a finished product is larger than the height hll of the entire semifinished product 11 (hll Next, the process of warm forging in the second step will be described in detail by reference to Fig. 1 to Figs. 5(a) to 5(d). Fig. 1 shows a press device PR used in the second step. The press device PR is a rotary press device, and its constitution is publicly known. Thus, its constitution will be described in detail merely in relation to a die set (DS) which has a structure characteristic of Embodiment 1 of the present invention. The die set DS is composed of a plurality of upper punches P from which works W hang; a plurality of dies D where the works W are inserted and shaped; a ram R and an upper ram UR for pressing the plurality of upper punches P; a press bed B where the plurality of dies D are fixed; and four guide posts GP which expand and contract. Each time the ram R rotates through a constant angle, the corresponding positions of the plurality of upper punches P and the plurality of dies D shift one by one. In this case, it does not matter whether the ram R is rotated clockwise or rotated counterclockwise in plan view, but in Embodiment 1, the ram R is rotated clockwise when viewed in plan. That is, the punch P inserts the work W into the die D and shapes it there. When the ram R ascends, the ram R rotates clockwise, in plan view, through a constant angle and stops. Thus, the punch P is located directly above the next die D. In this state, the punch P inserts the work W into the next die D and shapes it there. When the ram R ascends, the ram R rotates clockwise through a constant angle in plan view and stops. The rotary press device, which performs shaping in this manner, is a publicly known technology, so that an explanation for the rotating mechanism will not be offered any more. The plurality of upper punches P hanging the works W correspond to "fixtures for fixing the works" described in the solution means 2. The plurality of dies D include "fixtures for fixing the dies" described in the solution means 2. The plurality of dies D and the plurality of upper punches P incorporate heaters (not shown), which can hold the plurality of dies D and the plurality of upper punches P in a constant temperature state at any temperature between room temperature (10°C to 30°C) and 870°C. Since the dies and punches equipped with the heaters are publicly known, a detailed explanation for them is omitted. The reason for the limitation on the temperature range is as already presented herein. The whole of the plurality of dies D and the plurality of upper punches P is surrounded by an outer cylinder 4 and an inner cylinder 5 comprising a heat insulating material (see Fig. 3). That is, a double cylinder composed of the outer cylinder 4 and the inner cylinder 5 forms a doughnut-shaped space C1, and the plurality of dies D and the plurality of upper punches P, as a whole, are embraced within the space C1. A part or all of the ram R is formed from a heat insulating material, and a part of the heat insulating material portion of the ram R is configured in a cylindrical shape as a shielding tube 6 positioned inwardly of the inner cylinder 5. The outer cylinder 4, the inner cylinder 5, and the shielding tube 6 are configured to correspond to "heat insulating wall" described in the solution means 2. A heat insulating layer HS comprising a heat insulating material is interposed between the ram R and the upper ram UR. A heat insulating layer is also provided between the plurality of dies D and the press bed B, although this is not shown. These heat insulating layers also correspond to the "heat insulating wall" described in the solution means 2. A dish-shaped float 7 is provided in a lifted state in a space C2 inside the inner cylinder 5. The float 7 has a lowermost position determined by a plurality of protrusions 5a provided on the inner cylinder 5. A plurality of airways Al are bored in the inner cylinder 5, and the space C1 and the space C2 are in communication by the plurality of airways Al. Moreover, a plurality of airways A2 are bored in the ram R as well, and a space C3 above the float 7 and the outside space are brought into communication by the plurality of airways A2. A rectangular window portion 41 is formed in a front part of the outer cylinder 4 by boring (see Fig. 2). A door DR is mounted on a front part of the ram R, and is adapted to shut the window portion 41 of the outer cylinder 4 as the ram R is lowered. The numeral 42 denotes an air curtain device having a plurality of blowoff ports 42a bored and arranged parallel in an upper part thereof, and the plurality of blowoff ports 42a are disposed parallel along the lower side of the window portion 41. Next; the actions of the die set DS will be described. The press device PR is used in the second step in the manufacturing method of Embodiment 1 of the present invention. Thus, an explanation for the actions of the die set DS serves, unchanged, as an explanation for the second step in the manufacturing method of Embodiment 1 of the present invention. The semifinished product 11 for the valve umbrella portion is carried into the die set DS by a carry-in device (not shown). This carry-in act is performed through the window portion 41. On this occasion, the ram R is in an ascending state as shown in Fig. 2. The semifinished product 11 is carried, as the work W, into the space CI, with the expanded-diameter section 111 directed upward (direction a in Fig. 4b), and is suspended from and fixed to a hanger H (see Figs. 4a, 4b) of an upper punch PI (P) in the shape of a horseshoe in bottom view. The hanger H is a part of the "fixtures for fixing the works" described in the solution means 2. Fig. 4b is a bottom view of the upper punch P1I (P). In a state where the work W (semifinished product 11) has been carried in, the die D is not present below the upper punch P1 (P). When the work W (semifinished product 11) has been completely suspended from and fixed to the hanger H of the upper punch P1 (P), the carry-in device (not shown) recedes from the window portion 41, and the ram R rotates through a constant angle clockwise in plan view. At this time, the center of the work W (semifinished product 11) lies directly above the center of the die D1, and the rotation of the ram R is stopped here (see Fig. 4c). Then, the ram R lowers (direction X in Fig. 4c). When the ram R descends, the work W (semifinished product 11) is inserted into the die Dl (D), whereupon first drawing is performed. Then, the ram R ascends (direction Z in Fig. 4c). Further, the ram R rotates through a constant angle clockwise in plan view and stops directly above a die D2 (D) (see Fig. 4d). Then, the ram R lowers (direction X), and the work W undergoes second drawing by the die D2 (D). In the above-described manner, the work W is subjected to drawing until it reaches a die DN (D) in Fig. 5b, whereby it is shaped as a valve umbrella portion 1 as a finished product (see Fig. 5c). Fig. 5a shows a state during the process where the work W is located directly above a die DM (M