An apparatus for accurately detecting extraneous matter, such as cloth fragments, colored cotton fibers or seeds, which is contained in raw cotton and different from raw cotton in color. The extraneous matter different in color from the raw cotton is detected by a sensor (34) opposed to a colorless transparent wall portion (36) defining a passage (35) for transporting the raw cotton by an air stream. The outlet of the passage is contin­ually opened and closed by a shutoff valve (33) for the air stream to transport the raw cotton intermittently and for the sensor to detect the extraneous matter while the cotton is at rest.