PURPOSE: To supply a sufficient amount of steam and to easily eliminate clogging by introducing steam into a split inner mold through the gap between the constitutional members of the split inner mold and the peripheral gap of a raw material filling device.

CONSTITUTION: A female inner mold 5 and a male inner mold 6 are provided to a female mold frame 1 and a male mold frame 2. Steam chambers 8, 9 are respectively formed between the female mold frame and the female inner mold 5 and between the male mold frame 2 and the male inner mold 6 and the steam from a steam valve 15 is introduced into a cavity 7 while the steam from the steam valve 16 is introduced into the cavity 7 and the steam from a steam valve 19 is introduced into the cavity 7 through the mating surface between the respective first inner molds 5a, 6a of the female and male inner molds 5, 6. Since steam is supplied through gaps 11, 12, 14 and the mating surface of both of the first inner molds 5a, 6a, a sufficient amount of steam can be supplied.