PURPOSE: To enable cutting-off of a gate part from plunger tip and to prevent breakage of sprue part in a product by retreating once the plunger tip for pushing out the sprue part from an injection chamber at the time of opening dies under condition of the die clamping.

CONSTITUTION: At the time of opening the dies, the plunger tip 14 is once retreated under condition of the die clamping, proceeded advancing of the plunger tip 14 for pushing out the sprue part 16. By this method, shapes of the front end part of the plunger tip 14 and in front end part of a sleeve 12 enable low pressure casting and on the other hand, even in the case of constitution, in which sticking between the plunger tip 14 and the sprue part 15 easily occurs, the sprue part 15 can be surely cut off from the plunger tip 14. Therefore, the breakage of the gate part 17 in the product 18 is perfectly prevented and the low pressure casting can be executed.