PURPOSE: To improve a transfer property and strain by shortening a pressure raising time when an injection process is transferred to a compression process, by respectively pressurising both of the front and rear chambers of a mold clamping ram during the injection process by separate pressure sources and releasing the pressure of the mold clamping ram when the injection process is transferred to the compression process.

CONSTITUTION: When a movable platen 2 reaches a set position, said platen is stopped temporarily and the hydraulic oil of a hydraulic source 34 is introduced into a chamber-B and the hydraulic oil of a hydraulic source 35 is introduced into an chamber-A and the outputs of both oil chambers are allowed to be well balanced to perform positional control. Therefore, strain due to flow resistance is reduced. When a predetermined amount of a resin is injected, the pressurizing force of the chamber-A is increased while pressurization is applied to the chamber-B to allow the movable platen to advance by difference pressure. whereupon, the lowering in the spreading speed of the leading end part of the molten resin with the advance of the molten resin is relaxed and force right-angled to the flow direction of the molten resin is applied to said resin and, therefore, the orientation of the resin in the flow direction is reduced. When the completion of mold closing and filling is detected, the pressure of the chamber-B is released and the force negated by the output of the chamber-B is added to the chamber-A to perform the raising of pressure to compression pressure rapidly and, therefore, a transfer property can be enhanced.