PURPOSE: To accurately discriminate a color by a method wherein cullet to be treated is supplied to a color discriminating belt conveyor while the particle size thereof is arranged and the center part of all of the cullet is grasped by a color discriminating sensor.

CONSTITUTION: Cullet is arranged in its size by a rotary type dimension sorting apparatus l to be successively supplied to color discriminating belt conveyors 16, 25, 33, 42. Slits are formed to the center parts in the lateral direction of the endless annular belts 19, 28, 36, 45, used in the color discriminating belt conveyors 16, 25, 33, 42 over the entire peripheries thereof, and color discriminating sensors 21, 29, 37, 46 are arranged so that emitted lights pass through the slits are provided to the midway parts near to the front end parts in the advance direction of the color discriminating belt conveyors 16, 25, 33, 42. Therefore, the center part of the fed cullet is necessarily grasped by the color discriminating sensors 21, 29, 37, 46. By this method, a color can accurately be discriminated.