PURPOSE: To shorten cycle time by a method wherein mold clamping force by a mold clamping apparatus is released by the time when cooling of a molded item is finished after finish of feeding of a resin and the mold is opened after cooling is finished.

CONSTITUTION: When a time set in a cooling timer TR2 in advance passes, as a contact point TR2a is closed, a mold opening solenoid OP is energized and a solenoid D of an electromagnetic valve DV1 is turned ON. Therefore, a hydraulic oil for a hydraulic pump is sent to the rod side of a hydraulic cylinder 17 through pipes 22a and 22 to contract the hydraulic cylinder 17 and the hydraulic oil of the head side of the hydraulic cylinder 17 is returned to an oil tank 26 without passing through a throttling valve and through pipes 21, 21a, 24a and 24. A movable stage 3 is moved to the left by contract movement of the hydraulic cylinder 17 through toggle mechanism 2 to open molds 5 and 6. After mold opening is finished, a limit switch is made OFF. It is possible thereby to shorten a cycle time.