An injection system of a die casting machine giving the necessary casting pressure without adjusting the accumulator side, provided with an injection cylinder including an injection piston linked with the injection plunger and a booster piston arranged behind the injection piston, an accumulator for supplying a pressurized liquid of a predetermined pressure, a liquid pressure circuit for supplying the pressurized liquid from the accumulator to the injection cylinder, driving the injection piston, then driving the booster piston, a control valve for controlling a flow rate of the pressurized liquid discharged from the front side of the injection piston of the injection cylinder so as to control a flow rate of the injection piston, and controlling the pressure of the pressurized liquid at the front side of the injection piston by the timing of closing the control valve to determine the final value of the casting pressure.