PURPOSE: To distribute a predetermined thickness in a parison having a partly varied thickness by reducing a pressing force during moving of an inner core during molding, moving it to a predetermined position, and then pressing the parison by a pressing pressure specified after the movement.

CONSTITUTION: An injection hydraulic cylinder 1 is detected at its extruding pressure by a molten resin extruding pressure detector, input at the pressure to a parison program controller 10, and its command is controlled through an injection pressure regulator 7. On the other hand, an inner core 5 is relatively vertically moved to a die ring 6, detected at its position by an inner core position detector 3, and moved at its position by a command of the controller 10. The extruding pressure for moving the core 5 to a predetermined position during molding is lowered, and the parison is extruded by a pressing force specified after the core 5 is moved to the predetermined position. Thus, even when a parison having different thicknesses in a vertically circumferential direction is required, the parison having desired thickness structure can be formed.