A dispersing medium containing plural types of particles is let to flow in a flow path formed in a flow cell. The flow cell is irradiated with interfering light to form interference fringes of a pattern of stripes. In another arrangement, light is two-dimensionally scanned at high speed so as to cross a flow in the flow path. The moving particles receive a braking force by the light gradient force whenever they pass through each irradiation light stripe. In that case, greater braking forces act on particles having larger sizes (or larger refractive indices) than on particles having smaller sizes (or smaller refractive indices). Accordingly, the particles receiving smaller braking forces pass through the irradiated position more rapidly, so that particles can be separated to flow in the order from the particles receiving smaller braking forces to the particles receiving greater braking forces.