PURPOSE: To prevent the lowering of product strength by passing steam toward only the top part of the peripheral edge part of a biodegradable molding and preventing the structure of the entire region of the peripheral edge from receiving the effect of steam.

CONSTITUTION: Upper and lower molds 1a, 1b are mounted on mold stands 8a, 8b and a vertical gap S1 made uniform in width and a horizontal gap S2 made uniform in width are provided between the upper and lower molds 1a, 1b from the top part 9b of the peripheral edge part 9 of the lower mold 1b. Inverted cone-shaped steam venting grooves S3 are formed to the vertical gap S1 here and there. The steam venting grooves S3 can be formed into an inverted trigonal pyramid shape or an inverted truncated cone shape and the number of the steam venting grooves S3 formed to the vertical gap S1 or the size thereof is allowed to correspond to the amt. of steam generated from the raw material housed in a housing space 7. Steam goes toward the top part 9b of the peripheral edge part 9 of the lower mold 1b to flow as shown by an arrow (a) through the horizontal gap S2 to pass the top part of the peripheral edge part of the molding baked in the housing space 7 without extending to the expanded par 9a of the lower mold.