PURPOSE: To continuously use a hydraulic fluid safely and economically by inhibiting the growth of microbe, by by-passing part of a hydraulic fluid composed of a neutral aq. glycol soln. during circulation for use, stevilizing it and returning it to a storage tank.

CONSTITUTION: A hydraulic fluid (neutral aq. glycol soln.) in a tank 1 is fed through an absorption tube 2 to a high-pressure cylinder 6 by means of a high- pressure pump 3, and returned through an outlet 11 to the tank 1. During this operation, a piston head 8 is vertically moved through a rod 7 to prepare a rolled product between the head 8 and a mold 9. In the above system, part of the hydraulic fluid in the tank 1 is taken out through a suction pipe 19, fed to a sterilizer 22 (e.g. by ultraviolet light irradiation) to sterilize it and returned through a discharge tube 23 to the tank 1. According to the above method, it is not necessary to make the hydraulic fluid alkaline, so that the formation of sludge caused by saponification due to the penetration of lubricating oil does not occur and stable operation becomes possible over a long period of time.