PURPOSE: To obtain a constant clamping stroke to prevent tie bars from being elongated and deflected or a clamping piston from falling down at the topmost position thereof by a method wherein a movable platen and an adjusting device for adjusting the mold thickness are set on the tie bars provided between a fixed platen and a clamping cylinder support.

CONSTITUTION: On four tie bars 4 provided between a fixed platen 3 and a clamping cylinder support 2, a movable platen 5 is supported. Male screws 4a are provided on the tie bars 4 on the side of the clamping cylinder support 2. A clamping cylinder provided with a piston 10 is provided on the rear part of the clamping cylinder support. The clamping cylinder support 2 can be moved relatively to the tie bars 4 by a moving device 30 provided with adjusting nuts 31 fitted with the male screws 4a. In this manner, the the mold thickness is changed, a constant clamping stroke can be obtained only by adjusting the adjusting nuts 31.