PURPOSE: To prolong the service life of a die, to reduce the proportion defective of products and to make the large scale of a device unnecessary.

CONSTITUTION: A supporting part 42 for supporting a parting surface part of a moving side die 19 fitted to a moving side die plate 13 is arranged. The supporting part 42 supports the moving side die 19 by abutting a slope 44 arranged at the supporting part 42 on a slope 43 arranged at the moving side die 19. The slope 44 is linked with die clamping movement of a die clamping device 10 and abutted corresponding to separation between a guide rail 24 and a free roller 26A. By this method, posture deformation of the moving side die 19 is prevented and the long service life of the die can be obtd. without breakage of a guide pin 21 and a guide boss 22 and the proportion defective of the products can be reduced.