PURPOSE: To prevent the inside of pressed bodies from getting thinner by providing a weir that can move to the inner part of a mold in the squeezing parts of partitioned molds.

CONSTITUTION: When the molds 11, 12 are closed, the parison 6 is crushed gradually by the squeezing parts A, A. Further, just before the molds 11, 12 are closed, the cam surface 15a of the slide cam 15 abuts upon the drive cam surface 18a of the drive cam 18. Hereby, while the slide cam 15 compresses the elastic body 16, it moves toward the lower part in the cam-containing chamber 14. At this time, since the weir 17 being installed so that the squeezing part A is crossed with the movement of said cam 15 moves downward, the lower side of the parison 6 put between both compressed parts A, A is forced down to supply a sufficient quantity of resin to the place relevant to the inside of the compressed parts 7a in a molded piece. After blow molding, said compressed parts 7a take a sufficient thickness of resin.