PURPOSE: To prevent a molded rib from inclining and occurrence of deformation such as strain by blowing a fluid in a parison under a condition wherein cores are forwarded and projected to a molding face side and thereafter retreating successively each divided core at least by two stages.

CONSTITUTION: Cores 2a and 2b are forwarded by a specified amt. in the inside of a mold 1a and a compressed air is blown therein. A parison P is tightly adhered to the inner face of molds 1a and 1b and the cores 2a and 2b in such a way that it is tightly pressed against them to all the corners. Then, before the parison P is solidified, the cores 2a and 2b are successively retreated. At first, a free end part 2a' of the core 2a is retreated by actuating a hydraulic cylinder 6a in such a way that it is made at the same plane as the inner face of the mold 1a. Then, by actuating another hydraulic cylinder 6b, the remaining core 2b is retreated in the same way as the core 2a. As the result, a rib is formed by cores 2a and 2b divided into two. In this case, a synthetic resin at a position integrally press-adhered by the previously retreated core 2a supports a synthetic resin at a position of the afterward retreating core and prevents it from deformation such as inclination thereof.