PURPOSE: To switch the air pressure of air systems in multiple stages as required by providing a plurality of air supply systems respectively having selective air supplying solenoid switching valves, and electromagnetic proportional control valves for controlling supply air pressures to the systems.

CONSTITUTION: A microcomputer 52 sequentially variably sets the secondary side pressure of an electromagnetic proportional control valve 51 at the time of a predetermined timing upon progress of a molding process to a preset pressure. That is, pressures of the air supplied from a main air supply system 25 to air supply systems 20-24, and electromagnetic switching valves 31-35 are selectively controlled to be opened and closed in response thereto. Accordingly, the air of predetermined pressure responsive to each step is supplied selectively to a filler 8, a stationary side vapor chamber 3, a movable side vapor chamber 6 for predetermined time, and preblowing, bead filling, blow-back draining, mold releasing, and air supplying of an ejector are executed. Thus, the air pressures of the air supply systems can be controlled to be switched in multiple stages as required.