A first direction shifting valve adapted to supply pressurized hydraulic oil to a hydraulic chamber on the rearward displacement side and discharge hydraulic oil from the hydraulic chamber on the rearward displacement side, and moreover, discharge hydraulic oil from a hydraulic chamber on the forward displacement side is disposed in a hydraulic path extending between a hydraulic pump and an injection cylinder. In addition, a second direction shifting valve adapted to supply pressurized hydraulic oil to a chamber on the forward displacement side is disposed in the foregoing hydraulic path. Additionally, a third direction shifting valve is disposed to build a bleeding-off circuit in cooperation with the second direction shifting valve. While a hydraulic path on the rearward displacement side is blocked by the third direction shifting valve, a hydraulic path on the forward displacement side is hydraulically connected to the tank side. When the third direction shifting valve is shifted to the opposite side so that the hydraulic path on the rearward displacement side is hydraulically connected to the tank side and the hydraulic path on the forward displacement side is blocked by the third direction shifting valve, an injection molding operation can be started. With such construction, a rising time of each injection molding operation can be shortened while pressurized hydraulic oil is preliminarily exerted on the hydraulic chamber on the forward displacement side under the bleeding-off condition.