PURPOSE: To purify a lubricant liquid or a coolant liquid simply in a short time before the beginning of the operation of a machine tool or after the ending of the operation of the machine tool, by sucking the lubricant liquid or the coolant liquid, which is introduced into a sludge, bag, through a cushion tank to filter and recover it.

CONSTITUTION: A valve V1 is opened, a valve V2 is closed and a blower 9 is operated. A lubricant liquid or a coolant liquid contg. impurities within a tank 1 is sucked into a sludge bag 3 in a sealed state to store it. Then the valve V1 is closed, the valve V2 is opened, and at the same time, valves V3, V4 and V5 are closed. The stock liquid within the sludge bag 3 is filtered by means of a filter medium 5, and sucked into a cushion tank 7 where the liquid is stored. Then the valves V2 and V5 are closed, the valve V3 is opened, and a pump 12 is operated. When the hole size of the filter medium 5 is small, the valve V6 is closed, the valve V7 is opened and the filtrate within the tank 7 is directly fed to the tank 1 and recovered. When the hole size of the filter medium 5 is large, the valve V6 is opened, the valves V7, V8 and V9 are closed and the filtrate is re-filtered by means of a filter 15. Then, the filtrate is fed through a pipe 17 to the tank 1 and recovered.