PURPOSE: To obtain easily a vessel such as a bottle which is uniform in its thickness distribution, superior in transparency, glossy and beautiful by eliminating temperature irregularity in a preform of porypropylene, by setting up favorably a draft of the preform and the thickness distribution of the preform, in an injection mold.

CONSTITUTION: At the time of molding of a preform of polypropylene through injection molding and orientation blow molding of the preform into a molded product such as a vessel, a draft of the preform in an injection mold is made sufficiently large so as to fall within a range where high-temperature mold release of the preform is possible and a thickness distribution of the preform is set up beforehand so as to make the thickness distribution of a molded product favorable. A temperature irregularity is eliminated by making a temperature of the released preform uniform through a heat flow of potential heat of preform itself while the orientation blow molding is being performed. With this construction, the high-temperature mold release is facilitated and the preform free from the temperature irregularity can be obtained. Then as the thickness distribution of the preform is set up beforehand so that the molded product becomes an ideal thickness distribution, in the preform released from an injection mold, orientation blow molding of a molded product of thin uniform thickness is performed and molding of a vessel, which is uniform in thickness and transparent, becomes possible.