To accurately and stably supply a molten metal volume, and prevent the molten metal from sticking to the back of a piston by providing a molten metal cut operation fluid supplying pipe for prevention of the molten metal supply into a plunger sleeve from a molten metal supplying pipe after a piston in the plunger sleeve is driven forwardly and passed through the metal supplying pipe.

When a piston 10 is forwarded and passed through an opening 6a of a metal supplying pipe 6, a space S to be formed between the back of the piston 10 and a sealing material 12 is expanded and, the metal supplying pipe 6 and a metal cut operation fluid supplying pipe 7 are linked. The metal cut operation fluid pressing force P2 is supplied at a pressing force (a+α mmHg) higher than an operation pressing force P1 (a mmHg) for the molten metal 1 to be supplied from the metal supplying pipe 6. Therefore, the molten metal 1 is forced down from the opening 6a by the metal cut operation fluid pressing force P2, and the molten metal 1 supply from the metal supplying pipe 6 into the plunger sleeve 11 is prevented. Thereby, improvement of the product quality is achieved by preventing the molten metal from being supplied into the plunger sleeve 11 during injection, and the bubble remaining in the molten metal.