To simultaneously realize high speed opening/closing and a high pressure clamping of a mold by obtaining a high drive speed and a large drive force for a movable platen by means of a ball screw mechanism.

A first ball screw nut 32 and a cylindrical body 12 for opening/ closing a mold are integrally moved in an axial direction by rotating to drive a first ball and thread shaft 20 by a first drive means 24 and, in turn, a movable platen 14 fixed to the cylindrical body 12 is actuated to open/close the mold. On the other hand, second ball and screw shafts 34, 34 are rotated to drive by a second drive means 48 under such a state that a mold clamping member 40 and the cylindrical body 12 are engaged with each other by engaging means 64, 66 at a position where the mold is clamped by the movable platen 14. Thus second ball screw nut 36, 36, the mold clamping member 40 and the cylindrical body 12 are integrally moved in the axial direction to make the movable platen 14 clamp the mold.