To reduce the amount of waste liquid subjected to disposal treatment by extending the life of coating pretreatment liquid by removing an oil component dispersed in the coating pretreatment liquid over a long period of time.

The pretreatment liquid in a tank 10 is supplied to a filter apparatus 15 equipped with a filter element 18 to collect and remove impurities comprising a fine solid in the pretreatment liquid. The pretreatment liquid from which the fine solid is removed is supplied to an adsorption type filter apparatus 20 from the filter apparatus 15 to adsorb and remove impurities comprising the emulsified oil component dispersed in the pretreatment liquid. The pretreatment liquid from which the emulsified oil component is removed is returned to the tank 10 from the adsorption type filter apparatus. The filter apparatus 15 has a cleaning function for washing off impurities comprising the fine solid collected by the surface of the filter element 18.