To adjust the volume of a molten metal to be filled in a cavity to a given volume constantly after the injection velocity is appropriately switched to a high velocity by a simple control.

Total volume W of a molten metal supplied in a plunger sleeve 5 is detected, and a position P1 for switching a forwarding velocity of an injection plunger 6 is decided corresponding to the detected total volume W of the molten metal in a manner that the volume of the molten metal to be filled in the cavity 2 becomes constant after the molten metal is reached a level L at a switch position of the injection velocity. Then, when the injection plunger 6 engaged in the plunger sleeve 5 is forwarded to emit the molten metal to the cavity 2 in a mold 1, a forwarding velocity of the injection plunger 6 is switched at the decided position P1. Since the volume of the molten metal, to be emitted after the injection velocity is switched, which being set corresponding to various conditions for each molding, is adjusted to be kept constant, yield of the molding is improved.