A movable platen (4) capable of advance and retraction with respect to a fixed plate is provided with a tie bar connecting device (12) for meshing a split nut (18) with the multiple groove portion (16) of a tie bar (10), and a mold clamping cylinder (20) operable with the split nut (18) serving as a reaction force point. The mold clamping cylinder (20) comprises a primary piston (22) contacting the split nut (18) and a secondary piston (24) movable relative to the primary piston (22). It is arranged that the movable platen (4) is stopped just before the position at which the movable mold contacts the fixed mold, and in this state the split nut (18) is reliably meshed with the multiple groove portion (16). Thereafter, the secondary piston (24) and primary piston (22) are moved integrally with respect to the movable platen (4) so as to effect mold contacting and mold clamping. After molding, the secondary piston (24) and the primary piston (22) are relatively moved to effect mold release.