PURPOSE: To prevent air and thinner from infiltrating into a valve by stablty making the mixed fluid of air with washing liquid which is fine in air foams and very homogeneous as a whole, thus obtaining remarkably enhanced 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 n the case of color changeover of coating. In this case, the supply pressure of air is set higher than the supply pressure of washing liquid. Further, the opening time of the valve for washing liquid is regulated to the greater part in the whole washing time and also prolonged by several times in comparison with the opening time of the air valve.