To prevent such trouble as the leakage of oil from drained cooling water at the time of cooling a hydraulic cylinder unit, to keep the quality of pressure oil good and to prevent the development of fur and rust in a cooling piping etc.

A cooling chamber 5r is arranged so as to abut on a hydraulic chamber 5p in the hydraulic cylinder unit 2 for driving a die 1 in a casting machine through an O ring 8. A piston 6 is worked by feeding the working oil 10 into the hydraulic chamber 5p and also, cooling oil 14 having the same quality as the working oil 10 is introduced into the cooling chamber 5r to cool the cylinder rod 10 etc. This cooling oil 14 is taken out from a cooling tank 15 and after using to cooling, this oil is cooled with a cooler 20 and returned back to the cooling tank 15 and recycled. Further, the cooling tank 15 and the hydraulic tank 11 are connected with an over-flow piping 21.