To improve the amount of a collected product in a dispersion treatment of a pigment or a wax.

Prescribed amounts of the pigment, a dispersant solution and deionized water are fed to the interior of a tank 10, and the pigment is subjected to the dispersion treatment by using a disperser 80. The pigment-dispersed liquid obtained by the dispersion treatment is extracted from the dispersion treatment system, and a prescribed amount of the deionized water is fed to a dispersion treatment loop 70 again to clean the dispersion system. The cleaning liquid caused by the cleaning is recovered and used as a part of the product.