At the operation of a suction pump VP, a negative pressure is generated in a supply-side
pipe line 4, a cooling passageway 2a of a mold 2, and a return-side
pipe line 5. Cooling water stored in a water tank 3 is sucked by the negative pressure, flows from a first feed-water inlet 3a to the supply-side
pipe line 4, flows through the supply-side pipe line 4, and enters the cooling passageway 2a of the mold 2. After the cooling water cools down the mold 2 while flowing through the cooling passageway 2a of the mold 2, it flows out to the return-side pipe line 5. On the other hand, at the operation of the suction pump VP, a negative pressure is also generated in a bypass pipe line 6. The cooling water stored in the water tank 3 is sucked by the negative pressure, flows from a second feed-water port 3b to the bypass pipe line 6, and enters the return-side pipe line 5 via the bypass pipe line 6.