PURPOSE: To enhance the smoothness of a paint film and to reduce the irregularity thereof, by controlling the amount of the diluting solvent added to paint according to the shape and area of an object to be painted and performing automatic painting while adjusting the viscosity of the paint.

CONSTITUTION: A paint mixing valve 114 has function changing over not only the emission and recirculation of the high viscosity paint from a paint tank 119 but also the addition and non-addition of the diluting solvent from a diluting solvent tank 120 at the time of the emission of the paint. When an area having a complicated shape, where the dripping of a paint film must be prevented, like the local part of a car body is painted, the high viscosity paint from the paint tank 119 is supplied to a painting gun from the mixing valve 114 through a trigger damp valve. When a general part is painted, needle valves 8, 15 are operated and a required amount of the diluting solvent from the diluting solvent tank 120 is guided to a mixing chamber 11 to be mixed with a high viscosity paint and the resulting mixture is supplied to the painting gun as low viscosity paint.