Abstract: Injection pressure acting on a molten material inside a nozzle in an injection process also acts on a mould clamping piston disposed in an injection cylinder to face an injection piston. The injection pressure acting on the mould clamping piston is transmitted to an auxiliary platen mounted at the right ends of tie bars, thus producing tensile force in the tie bars. Because of the tensile force, a movable platen mounted at the left ends of the tie bars is pulled toward a stationary platen, which in turn presses a movable mould against a stationary mould. Consequently, clamping force proportional to the injection pressure acts on the moulds, thus firmly clamping the moulds.