PURPOSE: To recover oil, sludge, dirt, etc., stably by installing a reserve tank, in which the level of coolant changes only slightly, between the outlets of a coolant tank and a machine.

CONSTITUTION: A reserve tank 4, in which the level of coolant changes only slightly, is installed between the outlets of a coolant tank and a machine. Introduced coolant 3 flows vertically through layers partitioned by partition plates 5 of the reserve tank 4 to be sent into the coolant tank from an outlet 2. When a dirt receiver 6 is placed on the bottom of the reserve tank 4, sludge can be removed to a certain extent by lifting it quietly during the stop of a coolant pump so that the cleaning interval of the reserve tank 4 can be extended. Oil, dirt, etc., floating on the surface of the aqueous coolant adhere exclusively to an oil resistant roller which is rotated very slowly, while the coolant drips from the roller.