Abstract: A roller conveyer transports sheet material to accumulating hoppers provided with rotatable distributing chutes. Chute drivers are electrically connected to a unit for measuring the material thickness via a control unit. The thickness measuring unit comprises a radioisotope thickness gauge including a radiation detector and a radioactive source, positioned on opposite sides of the sheet material between two pairs of drive rollers of the conveyer. A meter measures the average value of the radiation intensity during a period of time within which a single sample of the material being sorted passes through the radiation zone. Another meter measures the instantaneous value of the radiation intensity.