To provide a blow molding device capable of improving the cut-off of a parison at a pinch off parT of the mold well, improving the molding efficiency and lowering the mold defect ratio.

A blow molding device comprises a pair of split molds facing each other formed of a vertically-extruded parison. A high speed hydraulic cylinder 4a for sliding a platen 1 for holding the split molds, for opening/closing and a high pressure hydraulic cylinder 4b are provided above and below. A hydraulic circuit is provided such that the high pressure hydraulic cylinder 4a is driven at a high speed to the direction for opening/closing the mold, and the high pressure hydraulic cylinder 4b is driven immediately before clamping the high pressure hydraulic cylinder 5a to the direction for closing the mold at a low speed with a high pressure.