PURPOSE: To eliminate the useless operating-stopping action of fluid supply means so as to enable the smooth level control of a storage tank by performing the machine operating number increase/decrease control of the fluid supply means corresponding to the fluid level of the storage tank.

CONSTITUTION: The filter operating number decrease level SP and filter operating number increase level ST of a storage tank (filtrate tank) 5 are set one point per each. When the level of the filtrate tank 5 reaches one of these levels, the operating number of fluid supply means (filters) 4a-4c is decreased or increased by one. The change rate of the filtrate tank levels is monitored, and the filtrate tank level is judged according to the grade of the change rate. In the case of the filtrate tank level being still the filter operating number decrease level SP or higher after decreasing the filter operating number, the filter operating number is further decreased by one. In the case of the filtrate tank level being still the filter operating number increase level ST or lower after increasing the filter operating number, the filter operating number is further increased by one. This results in eliminating the useless start-stop action of the filters 4a4c.