According to the present invention, even though the paint is filled or discharged repetitively to or from the paint bag (5), so that bursting of a coating bag (5) or lack of coating material during coating are not caused by accumulation of error between the filling amount and the using amount. That is, before a predetermined amount of the coating material is supplied to the coating material bag (5), the remaining coating material in the coating material bag (5) is extruded to empty it, since the coating material bag (5) is squashed by the pressure of fluid which was supplied to the hydraulic fluid chamber (6) outside of the bag.