To apply an ordinary furnace having simple structure as a holding furnace and to improve the supplying accuracy of molten metal by calculating a stroke for supplying the molten metal from the position corresponding to the fixed molten metal surface of a piston based on a level signal of the fixed molten metal surface, controlling an actuator so that the piston drives with this stroke and supplying the molten metal into an injection sleeve.

The fixed molten metal surface M is detected with a level sensor 23 and also, the stroke S2 for supplying the molten metal from the position L2 corresponding to the fixed molten metal surface of the piston 13 with a calculating means 25 based on the input of this detected signal. The actuator 21 is controlled with a control means 26 so that the piston 13 drives with this stroke to supply the molten metal M into the injection sleeve 6. In this way, even in the case of varying the molten metal surface in the molten metal holding furnace 16, the supply of the fixed quantity of the molten metal M can be attained. Then, by a molten metal supplying means 9, the ordinary furnace can be used as the holding furnace 16, and the fixed molten metal surface furnace is unnecessary. Further, since the rapid molten metal discharge can be executed by lowering the piston 13, the separated molten metal discharging mechanism is unnecessary.