FORM 2 THE PATENT ACT 1970 (39 of 1970) & The Patents Rules, 2003 COMPLETE SPECIFICATION (See Section 10, and rule 13) 1 . TITLE OF INVENTION METHOD FOR MANUFACTURING 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 The present invention relates to a method for manufacturing a hollow engine valve having a hollow portion formed from a stem part (body part) to a widening portion of a valve head part. BACKGROUND ART Various methods for manufacturing a hollow engine valve have been developed, and there have been methods which mold a hollow engine valve by forging. For example, Patent Document 1 discloses a method for manufacturing a valve head part of a hollow engine valve and a hollow engine valve. The manufacturing method described in Patent Document 1 obtains a hollow engine valve body by: molding a cup-shaped intermediate member by boring a columnar hole in the upper surface of a solid circular rod (a columnar billet) by hot forging with a punch; molding a valve head part forming part by widening a lower portion of the rod by forging; and performing necking on the rod multiple times to gradually narrow an upper portion of the valve head part forming part and thereby form a valve head part and a hollow stem part connected thereto. PRIOR ART DOCUMENT PATENT DOCUMENT Patent Document 1: Japanese Patent No. 4390291 (see [Embodiment!], [Fig. 1] to [Fig. 4], etc, for example) SUMMARY OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION In the method for manufacturing a hollow engine valve described above, the valve body is obtained by molding a semifinished hollow engine valve body as shown in Fig. 6. First as shown in Part A of Fig. 6, a semifinished hollow engine valve body 100 is prepared which includes a stem part (body part) 101 of a thickness to and a valve head part forming part 102 connected to thereto. In the semifinished hollow engine valve body 100, a hole portion 103 of a diameter do is formed from the stem part 101 to a widening portion of the valve head part forming part 102. Then, the first necking is performed, so that a stem part 104 has a thickness t1 (>to) and a hole portion 105 has a diameter di (t1) and a hole portion 107 has a diameter di (t(n-1)) and a hole portion 109 has a diameter dn (dl). Subsequently, the semifinished hollow engine valve body thus obtained by the above-described rotary swaging is subjected to necking. Specifically, the semifinished hollow engine valve body is subjected to necking which uses as many dies as the number of narrowing operations to thereby narrow the widening portion of the valve head part forming part and the stem part of the semifinished hollow engine valve body stepwisely. The dies are configured to press the widening portion and the stem part and have die holes with inner diameters decreasing slightly and successively according to the sequence of the operations. For example, in the first narrowing operation, necking is performed by using a die 61 as shown in Part C of Fig. 1. First, the die 61 is disposed above the semifinished hollow engine valve body. A columnar die hole 62 is formed in the die 61 from a lower surface portion 61a to an upper surface portion 61b. The die hole 62 has: a narrowing portion 63 which opens at the lower surface portion 61a and decreases in diameter toward the upper side; and a same diameter portion 64 which extends while maintaining the same diameter from a lower portion to an upper portion thereof. Because the narrowing portion 63 is formed opening at the lower surface portion 61a of the die 61, the stem part 14 of the semifinished hollow engine valve body can be smoothly guided into the same diameter portion 64 of the die 61. Thereafter, with the stem part 14 of the semifinished hollow engine valve body and the die hole 62 of the die 61 being arranged coaxially, the die 61 is pressed against the semifinished hollow engine valve body to narrow the stem part 14 of the semifinished hollow engine valve body with the die 61. As a result, the stem part 14 is stretched in the axial direction. Moreover, the stem part 14 becomes thicker than before the necking. The inner diameter and the outer diameter of the hole portion 15 in the stem part 14 become smaller than before the necking. Then, once the outer diameter of the stem part is reduced, the die is replaced with one matching the size of the outer diameter of this stem part, and this die is used to narrow the stem part. For example, in the n-th narrowing operation, necking is performed by using a die 65 as shown in Part D of Fig. 1. Note that n is a positive number equal to or greater than 2. First, the die 65 is disposed above the semifinished hollow engine valve body obtained by the (n-l)-th narrowing operation. Like the die 61, a columnar die hole 66 is formed in the die 65 from a lower surface portion 65a to an upper surface portion 65b. The die hole 66 has: a narrowing portion 67 which opens at the lower surface portion 65a and decreases in diameter toward the upper side; and a same diameter portion 68 which extends while maintaining the same diameter from a lower portion to an upper portion thereof. The die hole 66 is smaller in diameter than the die hole 62, and the same diameter portion 68 is formed smaller in diameter than the same diameter portion 64. Thereafter, with a stem part 16 of the semifinished hollow engine valve body and the die hole 66 of the die 65 being arranged coaxially, the die 65 is pressed against the semifinished hollow engine valve body to narrow the stem part 16 of the semifinished hollow engine valve body with the die 65. As a result, the stem part 16 is stretched in the axial direction. Moreover, the stem part 16 becomes thicker than before the necking. The inner diameter and the outer diameter of a hole portion 17 in the stem part 16 become smaller than before the necking. In other words, the above-described necking is performed until the outer diameter of the stem part of the semifinished hollow engine valve body reaches a predetermined size d1 and also until the inner diameter of the stem part reaches a predetermined size d2 (d2) and also the outer diameter reaches near the predetermined size (>dl) as a result of performing the necking on the semifinished hollow engine valve body, rotary swaging is performed with a core set inside the stem part. This process uses a core that matches the size of the inner diameter of the stem part of the semifinished hollow engine valve body adjusted by the necking. Thereby, the outer diameter of the stem part is reduced while the size of the inner diameter is maintained as is. The rotary swaging using the core is performed until the outer diameter of the stem part of the semifinished hollow engine valve body reaches the predetermined size dl. As a result, a hollow engine valve body is obtained. In this embodiment, the rotary swaging using the core is performed in eighth step S8. Subsequently, the length of the stem part of the hollow engine valve body thus obtained by the above-described rotary swaging is adjusted as needed. Then, as in the method for manufacturing a hollow engine valve according to the first embodiment described above, the tip (upper end) of the stem part of the hollow engine valve body is sealed. As a result, the hollow engine valve (finished product) can be obtained. Thus, the method for manufacturing a hollow engine valve according to this embodiment can achieve the same operation and effect as those by the method for manufacturing a hollow engine valve according to the first embodiment described above. In addition to this, because rotary swaging is performed after performing necking, it is possible to adjust the outer diameter of the stem part of the semifinished hollow engine valve body to a desired size. Furthermore, it is possible to improve the processing accuracy of the inner diameter of the stem part of the hollow engine valve as compared to methods for manufacturing a hollow engine valve in which necking is performed in the last step. Embodiment 4 A method for manufacturing a hollow engine valve according to a fourth embodiment of the present invention will be described with reference to Figs. 1 and 2. In Fig. 2, the crosses indicate the size of the outer diameter of a stem part of each of a semifinished hollow engine valve body and a hollow engine valve body according to Embodiment 4. The triangles indicate the size of the inner diameter of the stem part of each of the semifinished hollow engine valve body and the hollow engine valve body, and the long dashed short dashed line indicates Embodiment 4. In this embodiment, a semifinished hollow engine valve body is subjected to necking and then subjected to rotary swaging (rotary plastic working). Specifically, as shown in Fig. 2, a semifinished hollow engine valve body is subjected to necking in first step S1 to sixth step S6 (first necking step) and then subjected to rotary swaging using a core in seventh step S7 to eighth step S8 (rotary plastic working step). A semifinished hollow engine valve body is prepared as in the method for manufacturing a hollow engine valve according to the first embodiment described above. As shown in Part A of Fig. 1, a semifinished hollow engine valve body 10 includes a stem part (body part) 11 and a valve head part forming part 12a connected to a lower end portion of the stem part 11. A columnar hole portion 13 is formed from the stem part 11 to a widening portion of the valve head part forming part 12a. First, the semifinished hollow engine valve body is subjected to necking as in the method for manufacturing a hollow engine valve according to the first embodiment described above. Specifically, the semifinished hollow engine valve body is subjected to necking which uses as many dies as the number of narrowing operations to thereby narrow the valve head part forming part and the stem part of the semifinished hollow engine valve body stepwisely. The dies are configured to press the valve head part forming part and the stem part and have die holes decreasing in diameter slightly and successively according to the sequence of the operations. As the dies, dies 61, 65 shown in Parts C and D of Fig. 1 or the like are used, for example. This process is performed until the inner diameter of the stem part of the semifinished hollow engine valve body reaches near the predetermined size d2 (>d2) and also until the outer diameter reaches near a predetermined size (>dl). In this embodiment, the necking is performed in first step SI to sixth step S6. Subsequently, the semifinished hollow engine valve body thus obtained by the above-described necking is subjected to rotary swaging with a core set inside the stem part. This process uses a core that matches the size of the inner diameter of the stem part of the semifinished hollow engine valve body adjusted by the necking. Thereby, the outer diameter of the stem part of the semifinished hollow engine valve body is reduced while the size of the inner diameter is maintained as is. The rotary swaging using the core is performed until the outer diameter of the stem part of the semifinished hollow engine valve body mentioned above reaches the predetermined size d1 (>d2). As a result, a hollow engine valve body is obtained. In this embodiment, the rotary swaging using the core is performed in seventh step S7 to eighth step S8. Subsequently, the length of the stem part of the hollow engine valve body thus obtained by the above-described rotary swaging is adjusted as needed. Then, as in the method for manufacturing a hollow engine valve according to the first embodiment described above, the tip (upper end) oi the stem part o£ the hollow engine valve body is sealed. As a result, the hollow engine valve (finished product) can be obtained. Thus, in the method for manufacturing a hollow engine valve according to this embodiment, rotary swaging is performed after necking is performed to change the inner diameter of the stem part to a desired size. Therefore, it is possible to adjust the outer diameter of the stem part of the hollow engine valve to a desired size. Moreover, it is possible to improve the processing accuracy of the inner diameter of the stem part of the hollow engine valve as compared to methods for manufacturing a hollow engine valve in which necking is performed in the last step. Embodiment 5 A method for manufacturing a hollow engine valve according to a fifth embodiment of the present invention will be described with reference to Fig. 2. In Fig. 2, the crosses indicate the size of the outer diameter of a stem part of each of a semifinished hollow engine valve body and a hollow engine valve body according to Embodiment 5. The triangles indicate the size of the inner diameter of the stem part of each of the semifinished hollow engine valve body and the hollow engine valve body, and the long dashed double-short dashed line indicates Embodiment 5. In this embodiment, necking, rotary swaging (rotary plastic working) using a core, and necking are performed in this described order. Specifically, as shown in Fig. 2, a semifinished hollow engine valve body is subjected to necking in first step S1 to third step S3 (first necking step), then subjected to rotary swaging using a core of a predetermined diameter (>d2) in fourth step S4 to fifth step S5 (rotary plastic working step), and thereafter subjected to necking in sixth step S6 to eighth step S8 (second necking step). More specifically, in this embodiment, necking is performed in first step SI to third step S3 as in first step SI to sixth step S6 in the method for manufacturing a hollow engine valve according to the fourth embodiment described above, and rotary swaging using a core is performed in fourth step S4 to fifth step S5 as in seventh step S7 to eighth step S8 in the fourth embodiment described above. Thus, description of these is omitted. In this embodiment, once the inner diameter of the stem part of the semifinished hollow engine valve body reaches a predetermined size (>d2) and also the outer diameter reaches a predetermined size (>dl) as a result of performing the necking and then the rotary swaging on the semifinished hollow engine valve body, necking is performed on the semifinished hollow engine valve body thus obtained by the rotary swaging, as in the method for manufacturing a hollow engine valve according to the first embodiment described above. Specifically, the semifinished hollow engine valve body is subjected to necking which uses as many dies as the number of narrowing operations to thereby narrow the valve head part forming part and the stem part of the semifinished hollow engine valve body stepwisely. The dies are configured to press the valve head part forming part and the stem part and have die holes decreasing in diameter slightly and successively according to the sequence of the operations. This process is performed until the outer diameter of the stem part of the semifinished hollow engine valve body reaches the predetermined size dl and also until the inner diameter of the stem part reaches the predetermined size d2 (d2) and also the outer diameter reaches a predetermined size (>dl) as a result of performing necking on the semifinished hollow engine valve body, rotary swaging is performed with a core set inside the stem part. Specifically, as shown in Parts A and B of Fig. 3, a core 56 is set inside a hole portion 23 of a stem part 21 of the semifinished hollow engine valve body having been subjected to the necking. Then, dies (swaging die) 55 are caused to strike the portion of this stem part 21 other than the upper portion thereof while the core 56 is rotated in a direction R about the axis of the semifinished hollow engine valve body. As a result, the outer diameter of the stem part 21 is reduced while the size of the inner diameter is maintained as is, except the upper portion. On the other hand, the outer diameter of the upper portion of the stem part 21 is maintained at the size before the process and therefore serves as a large diameter portion 22. Note that the inner diameter of the upper portion of the stem part 21 is also maintained at the size before the process. Subsequently, once the outer diameter of the portion of the stem part 21 of the semifinished hollow engine valve body other than the upper portion reaches the predetermined size dl, the large diameter portion 22 of the stem part 21 is pressed in the directions of arrows Y toward the axis of the stem part 21 in a sealing step as shown in Part A of Fig. 4. As a result, as shown in Part B of Fig. 4, the large diameter portion 22 is pressed and becomes an end sealing portion 24 sealing the tip of the stem part 21. Thus, the method for manufacturing a hollow engine valve according to this embodiment can achieve the same operations and effects as those by the methods for manufacturing a hollow engine valve according to the third embodiment and the fourth embodiment. In addition to this, because the end of the stem part 21 can be closed by a series of processing operations, it is not necessary to separately prepare a member to close the end of the stem part 21. Accordingly, the manufacturing steps can be simplified. Note that although the foregoing first to sixth embodiments have described cases where rotary plastic working is rotary swaging, it is possible to use spinning or to use both rotary swaging and spinning as the rotary plastic working. In these cases, too, the same operation and effect as those by the methods for manufacturing a hollow engine valve described above can be achieved. For example, in the rotary plastic working step in the method for manufacturing a hollow engine valve according to the first embodiment described above, spinning may be performed as shown in Fig. 5, instead of the rotary swaging. Specifically, as shown in Part B of Fig. 5, a core 54 is inserted in the hole portion 13 of the semifinished hollow engine valve body 10. Then, a die (spinning die) 53 is pressed against the outer peripheral portion 11a of the stem part 11 while the semifinished hollow engine valve body 10 and the core 54 are rotated together in a direction R about the axis of the semifinished hollow engine valve body 10. As a result, the outer diameter of the stem part of the semifinished hollow engine valve body can be reduced while the size of the inner diameter is maintained as is. INDUSTRIAL APPLICABILITY By the method for manufacturing a hollow engine valve according to the present invention, it is possible to manufacture a hollow engine valve having a stem part of a desired thickness relatively easily. Thus, the method for manufacturing a hollow engine valve according to the present invention can be utilized beneficially in the automotive industry and the like. EXPLANATION OF REFERENCE NUMERALS 10 SEMIFINISHED HOLLOW ENGINE VALVE BODY 11 STEM PART 12a-12c VALVE HEAD PART FORMING PART 12d VALVE HEAD PART 13 HOLE PORTION 14,16,18 STEM PART 15,17,18 HOLE PORTION 21 STEM PART 22 LARGE DIAMETER PORTION 23 HOLE PORTION 24 END SEALING PORTION 31 SEALING MEMBER 51 DIE 52 CORE (MANDREL) 55 DIE 56 CORE (MANDREL) 61,65 DIE 62, 66 HOLE PORTION 63, 67 NARROWING PORTION 64, 68 SAME DIAMETER PORTION WE CLAIM: 1. A method for manufacturing a hollow engine valve having a hollow portion formed from a stem part to a widening portion of a valve head part, characterized in that the method comprises: a first rotary plastic working step of performing rotary plastic working on a semifinished hollow engine valve body which includes a stem part and a valve head part forming part connected to the stem part, to thereby narrow the stem part, the semifinished hollow engine valve body having a columnar hole portion formed from the stem part to a widening portion of the valve head part forming part; a necking step of, after the first rotary plastic working step, performing necking on the semifinished hollow engine valve body to thereby reduce an outer diameter and an inner diameter of the stem part, the necking using as many dies as the number of narrowing operations to thereby narrow the widening portion of the valve head part forming part and the stem part of the semifinished hollow engine valve body stepwisely, the dies being configured to press the widening portion and the stem part and having die holes with inner diameters decreasing slightly and successively according to a sequence of the operations; and a sealing step of, after the necking step, sealing a tip of the stem part to thereby obtain the hollow engine valve. 2. The method for manufacturing a hollow engine valve according to claim 1, characterized in that the rotary plastic working is any one of: rotary swaging including inserting a core inside the hole portion of the semifinished hollow engine valve body, and striking an outer peripheral portion of the stem part with a swaging die while axially rotating the semifinished hollow engine valve body and the core together; and spinning including inserting a core inside the hole portion of the semifinished hollow engine valve body, and pressing a spinning die against the outer peripheral portion of the stem part while axially rotating the semifinished hollow engine valve body and the core together. 3. The method for manufacturing a hollow engine valve according to claim 1, characterized in that the rotary plastic working is a combination of: performing any one of rotary swaging including striking an outer peripheral portion of the stem part with a swaging die while axially rotating the semifinished hollow engine valve body, and spinning including pressing a spinning die against the outer peripheral portion of the stem part while axially rotating the semifinished hollow engine valve body; and performing, after the any one of the rotary swaging and the spinning, any one of rotary swaging including inserting a core inside the hole portion of the semifinished hollow engine valve body, and striking the outer peripheral portion of the shaft part with a swaging die while axially rotating the semifinished hollow engine valve body and the core together, and spinning including inserting a core inside the hole portion of the semifinished hollow engine valve body, and pressing a spinning die against the outer peripheral portion of the stem part while axially rotating the semifinished hollow engine valve body and the core. 4. The method for manufacturing a hollow engine valve according to claim 1, characterized in that the method further comprises a second rotary plastic working step of performing rotary plastic working on the semifinished hollow engine valve body after performing the necking thereon, to thereby reduce the outer diameter of the stem part while maintaining the size of the inner diameter thereof as is. 5. The method for manufacturing a hollow engine valve according to claim 4, characterized in that the rotary plastic working in the second rotary plastic working step is any one of: rotary swaging including inserting a core inside the hole portion of the semifinished hollow engine valve body, and striking an outer peripheral portion of the stem part with a swaging die while axially rotating the semifinished hollow engine valve body and the core together; and spinning including inserting a core inside the hole portion of the semifinished hollow engine valve body, and pressing a spinning die against the outer peripheral portion of the stem part while axially rotating the semifinished hollow engine valve body and the core together. 6. The method for manufacturing a hollow engine valve according to claim 4, characterized in that the rotary plastic working in the second rotary plastic working step is performed only on a portion of the stem part other than an upper portion thereof, so that the outer diameter of the stem part is reduced while the size of the inner diameter thereof is maintained as is except the upper portion, whereas the outer diameter of the upper portion of the stem part is maintained at the size thereof before the rotary plastic, working to thereby form a large diameter portion, and in the sealing step, the large diameter portion is pressed to seal a tip of the stem part and thereby obtain the hollow engine valve. 7. A method for manufacturing a hollow engine valve having a hollow portion formed from a stem part to a widening portion of a valve head part, characterized in that the method comprises: a first necking step of performing necking on a semifinished hollow engine valve body which includes a stem part and a shaft head part forming part connected to the stem part and has a columnar hole portion formed from the stem part to a widening portion of the valve head part forming part, to thereby reduce an outer diameter and an inner diameter of the stem part, the necking using as many dies as the number of narrowing operations to thereby narrow the widening portion of the valve head part forming part and the stem part of the semifinished hollow engine valve body stepwisely, the dies being configured to press the widening portion and the stem part and having die holes with inner diameters decreasing slightly and successively according to a sequence of the operations; a rotary plastic working step of, after the first necking step, performing rotary plastic working on the semifinished hollow engine valve body to thereby narrow the stem part; and a sealing step of, after the rotary plastic working step, sealing a tip of the stem part to thereby obtain the hollow engine valve. 8. The method for manufacturing a hollow engine valve according to claim 7, characterized in that the rotary plastic working is any one of: rotary swaging including inserting a core inside the hole portion of the semifinished hollow engine valve body, and striking an outer peripheral portion of the stem part with a swaging die while axially rotating the semifinished hollow engine valve body and the core together; and spinning including inserting a core inside the hole portion of the semifinished hollow engine valve body, and pressing a spinning die against the outer peripheral portion of the stem part while axially rotating the semifinished hollow engine valve body and the core together. 9. The method for manufacturing a hollow engine valve according to claim 7, characterized in that the method further comprises a second necking step of, after performing the rotary plastic working on the semifinished hollow engine valve body, performing necking to thereby reduce the outer diameter and the inner diameter of the stem part, the necking using as many dies as the number of narrowing operations to thereby narrow the widening portion of the valve head part forming part and the stem part of the semifinished hollow engine valve body stepwisely, the dies being configured to press the widening portion and the stem part and having die holes with inner diameters decreasing slightly and successively according to a sequence of the operations. 10. The method for manufacturing a hollow engine valve according to claim 7, characterized in that the rotary plastic working is performed only on a portion of the stem part other than an upper portion thereof,, so that the outer diameter of the stem part is reduced while the size of the inner diameter thereof is maintained as is except the upper portion, whereas the outer diameter of the upper portion of the stem part is maintained at the size thereof before the rotary plastic working to thereby form a large diameter portion, and in the sealing step, the large diameter portion is pressed to seal a tip of the shaft part and thereby obtain the hollow engine valve.