To prevent the occurrence of a defective and the irregularity of quality in a mold assembly having a sprue.

A cavity 13 is formed between a pair of molds 10 and 12 which are made approach/separate from each other, the sprue 15 one end of which is opened to a seat surface 10a formed on the outer surface of one mold 10 and the other end of which is made to communicate with the cavity is formed. and a molten metal M injected from the tip of the nozzle 22 of an injection apparatus 20 brought into contact with the seat surface is packed in the cavity 13 through the sprue. A cooling medium control device 30 controls the flow rate of a cooling medium supplied to a cooling medium hole 16 formed in the vicinity of the seat surface so that the temperature in the vicinity of the seat surface of one end of the sprue detected by a temperature sensor 31 becomes a constant prescribed temperature. Preferably, the sprue is formed by the inner surface of another sprue bush 11 fixed in one mold.