A mold clamping apparatus for an injection molding machine comprises a stationary platen fixedly holding the stationary mold, a movable platen fixedly holding the movable mold and disposed opposite to the stationary platen, tie bars for guiding the movable platen for movement toward and away from the stationary platen, a movable platen driving mechanism for moving the movable platen along the tie bars to close and open the mold, a movable platen fixing mechanism for fixing the movable platen to the tie bars at a set position immediately before a position where the movable mold is joined to the stationary mold, and a clamping force applying mechanism that includes a sealed hydraulic cylinder actuator capable of converting energy of a hydraulic fluid into an intensified mold clamping force and of applying the intensified mold clamping force to the stationary platen. In one embodiment, the apparatus includes an electrically driven toggle mechanism which brings the movable and stationary molds together, at which point the hydraulic clamping force applying means applies significant clamping force to clamp the mold shut.