To provide a method for controlling a coating material by which the characteristics of a coating material (the physical properties of a coating material) can be maintained in a coating system wherein the coating material is circulated in the air for a long time.

The composition of volatile solvents in a coating material after an elapse of time is obtained by the calculation from the refractive index of a coating material immediately after the preparation thereof, the refractive index of each volatile solvent, and the refractive index of the coating material after an elapse of time. The volatile solvent composition evaporated with time is determined by the calculation from the obtained composition of volatile solvents in the coating material after an elapse of time and that immediately after the preparation of the coating material. A diluent having the same composition as that of the evaporated volatile solvent composition is used to adjust the solid content of the coating material after an elapse of time.