PURPOSE: To purify easily a lubricant or coolant solution containing impurities in a short time, to prolong their lives, and to reduce times to change the solutions, by filtering the solutions while being pressurized in a sludge bag.

CONSTITUTION: The blower 7 is driven, air in the sludge bag 3 and the suction pipe 2 is exhausted to the outside, a lubricant or a coolant solution contining impurities in the tank 1 is sucked from the pipe 2 into the sludge bag 3 in an airtight state and stored in it. The pressure pipe 10 is connected to the sludge bag 3, and the pipe 6 to the suction pipe 2 respectively, the blower 7 is droven and pressure is applied to the stored solution in the sludge bag 3, so that filtering action is promoted by the filtering material 5 of stored solution, and the purified filtrate is sent from the pipe 6 to the suction pipe 2 and recovered in the tank 1.