PURPOSE: To improve the durabilities of a plunger tip and a sleeve as well as to eliminate the poor run of molten metal and to improve the quality of product by supplying damped air to a cooling jacket around a casting sleeve to cool the sleeve.

CONSTITUTION: In the operation of casting a molten metal, water is jetted from a nozzle 41 to dry air in a tube 36 from a compressor, and the air containing moisture is discharged from an air outlet after passing through an air passage 35a. The gnawing between the inner peripheral surface of a casting sleeve 32 and a plunger tip 27a generating at the time of the vertical movement of plunger tip 27a is eliminated by reducing the deformation of sleeve 32 caused by the heat of molten metal by cooling the sleeve 32 with the aid of this moisture-containing air. At that time, the poor run of molten metal in a die due to the overcooling of sleeve 32 is prevented by controlling the water flow rate with the aid of a valve 39 to maintain the proper cooling temperature.