Abstract: Provided is a method for detecting objects to be sorted with an optical sorter that can detect lateral portions of objects to be sorted with a flat shape. A chute is provided with a plurality of parallel longitudinal grooves formed in the longitudinal direction by a plurality of elongated protruding walls, and objects to be sorted with a flat shape are allowed to flow downward on the surface of the chute such that substantially flat faces of the objects to be sorted touch the elongated protruding walls in the longitudinal grooves, and lateral portions of the objects to be sorted face the front-rear direction of the chute so that the optical detection unit detects the lateral portions of the objects to be sorted at the detection position. Preferably, the objects to be sorted with the flat shape are rice grains, and the optical detection unit detects bran remaining on back strings of the rice grains at the detection position.