A crosshead position Cf for obtaining a set mold clamping force is obtained based on a mold touch position Ct obtained in mold clamping process in immediately preceding molding cycle, and a crosshead is allowed to move to the position Cf. During that time, it is determined whether a load La of a servomotor for mold clamping operation which drives a toggle mechanism exceeds a judgment load Ls, and a crosshead position Ca when the load La exceeds the judgment load Ls is stored as a mold touch position. In the next molding cycle, a crosshead position Cf where the set mold clamping force is obtained is obtained based on this stored mold touch position.