PURPOSE: To clamp with high torque even by a small motor with small output by varying the number of effective cylinders for generating thrust by operating a selector valve and switching over the characteristic between the high speed and low thrust one and low speed and high thrust one.

CONSTITUTION: When a motor 1 is rotated in the state of mold opening, a ball thread nut 4 advances and pressure is generated in a cylinder 11. Thrust is not required in the initial stage of mold opening, and an electromagnetic selector valve 12 is set in the state shown in the drawing for increasing the closing speed. Then, the hydraulic pressure generated in the cylinder 11 is converted into thrust of a rod 5 by a cylinder 14 to move forward a movable platen 8. When the movable platen 8 reaches near the final position, the position of the platen 8 is sensed by a limit switch or the like, and the solenoid selector valve 12 is switched over automatically. Then, the hydraulic pressure generated in the cylinder 11 is fed to cylinders 13 and 14. At that time, the advancing speed of the rod 5 is reduced to 1/2, while its thrust is doubled to generate a large clamping force.