PURPOSE: To improve the safety by communicating a die clamping cylinder and a die opening cylinder in the initial die clamping period with a pressure relief port in an injection molding machine or the like to prevent the generation of a high die clamping force in the main ram.

CONSTITUTION: A pressure oil is applied to a boost cylinder 16 via a boost ram 11 to move a mobile platen 8. In the initial period of such a die clamping, if abnormality occurs in the equipment, a prefilling valve 17 is closed to allow a hyraulic pressure to work on a main ram 7 and move it by a high die clamping force. But at the same time, the pressure oil enters a die opening cylinder 24 via a pressure relief port 23 and circulates to an oil tank 12. Therefore, only a low die clamping force due to the boost cylinder 16 is generated and the movement of the platen 8 is checked safely with a detent 21 and a stopper 22. At the normal die clamping operation, with the movement of the main ram 7, the short-circuiting of the pressure relief port 25 is cut off. At the die opening, as the hydraulic pressure is fed to the die clamping cylinder 24, a ring- shaped cylinder 26 returns receiving pressure on the surface thereof and then, reaches the dead point gradually without impact by a momentum with the communication of both the ports 25 and 23.