PURPOSE: To improve workability, to reduce molding cycle time and to improve productivity by interposing a die height adjustig cylinder which adjusts the closing end limit of a mold opening and closing cylinder between a connecting member and movable platen.

CONSTITUTION: The movable platen and tie rod side are made freely slidable and the movable platen is moved forward and backward by moving forward and backward a piston rod 51 of a die height adjusting cylinder 49, by which the die height is adjusted with respect to the mold thickness. The movable platen is then moved at a high speed by the opening and closing cylinder 46 and is decelerated just prior to the die mating and thereafter the platen is further moved. The piston rod 51 of the cylinder 49 elongation fully and moves together with the connecting member and tie rods. The molds are mated when the movable platen and the tie rods are fixed by attaching and detaching means during the mid-way of the movement. The molds are further strongly clamped when oil is fed to a mold clamping cylinder 26. The pressure oil of the mold clamping cylinder is removed after the injection and thereafter the attaching and the detaching means are released. The molds are then opened by the mold opening and closing cylinder 46.