This invention is directed to a system for stirring pigment ink whose pigment particles in a container subside without moving the container containing pigment ink, and detecting that pigment particles have been stirred satisfactorily. To achieve this, a pair of electrodes greatly different in surface area and at least one of which has an inconstant width are arranged outside a container containing pigment ink whose pigment particles subside. An AC power supply is connected to the electrodes and applies an AC voltage to them. Then, an electric field generated between the electrodes causes induced polarization in pigment particles which subside in the container. The pigment particles are attracted to a portion having high electric field intensity. Convection occurs in the container, stirring the subsiding pigment particles. Based on a voltage detected by an interelectrode voltage detector, it is detected that pigment particles are dispersed sufficiently.