PURPOSE: To inhibit back flow due to the pressure difference of both valves by stably making the mixed fluid of air with washing liquid which is fine in air foams and very homogeneous as a whole, thus obtaining the remarkably enhance washing effect.

CONSTITUTION: The mixed fluid of washing liquid with air is made in a manifold by repeatedly opening and closing both a washing valve and an air valve respectively in the periodical moment of about 0.1-100 milli sec. Thereby the route of coating is washed in the case of color changeover of coating. In this case, in order to enhance the mixing action of air and thinner, the opening time of the valve for washing liquid is regulated to the greater part in the whole washing time. Further, the air valve is closed in a time for opening the valve of washing liquid and opened for a necessary period on a time for closing the valve of the same.