A paint cartridge is provided with a wash fluid passage in communication with a paint chamber, along with a wash fluid valve adapted to open and close the wash fluid passage. Provided on the side of a replenishing stool are a replenishing valve which supplies or discharges paint to or from the paint chamber, an extruding thinner feed/discharge valve which supplies or discharges paint-extruding thinner to or from an extruding thinner chamber, and a wash fluid supply valve which supplies a wash fluid to the paint chamber through the wash fluid passage. Accordingly, upon opening the replenishing valve, wash fluid supply valve and wash fluid valve, a wash fluid from the wash fluid supply valve is allowed to flow into the paint chamber through the wash fluid passage and discharged through a feed tube to wash away deposited paint therefrom.