PURPOSE: To form a baffle having thin thickness by moving a cooling pipe which holds a core material into the interior of a mold to form a baffle section and pulling out the cooling pipe during the curing of resin after blow molding, leaving the core material in the baffle section.

CONSTITUTION: Cooling pipes 22 holding a core material 24 on their ends are provided in a mold 21 and thereafter a heated parison is inserted thereinto and air is sent into the mold 21 after clamping it to expand the parison to a desired tank-like shape, and at the same time, the pipes 22 are slided by a specified distance to bond the ends of the baffles 13a, 13b to each other and then the air is stopped. Thereafter the mold 21 is cooled to cure resin, while at the same time water 23 as cooling medium is introduced into the outer channel of the pipes 22 to sufficiently cool and cure the resin of the baffles 13a, 13b. After the specified cooling, the cooling pipes 22 only are pulled out one after another, leaving core materials 24 in the baffles 13, 13b.