A mechanical safety device for an injection molding machine is provided with a laterally openable safety door on the flank of a mold opening/closing portion between a movable platen and a stationary platen. A rod is secured to the stationary platen or a base, and a rail, which ascends and descends as the safety door is closed and opened, is assembled to the rod. The rod is formed with a plurality of engaging step portions and tapered surfaces arranged alternately. On the other hand, a ratchet is rotatably supported on the movable platen. The rail ascends or descends to push up or down the ratchet, thereby locating the ratchet in a position where it engages with the engaging step portions of the rod or a position where it is disengaged from the engaging step portions.