The authors have made the experimental and the theoretical investigation of the screw conveyor. The theoretical values of the power and the efficiencies are in good agreement with the experimental ones, because the shape of the screw and the behavior of materials have been assumed to be close to the real phenomena to make the calculations easier. The experiments have revealed that : (1) There may be the optimum ratio of pitch to outer diameter of the screw, (2) the tip clearance must be reduced as much as possible and (3) the total efficiency decreases steeply, when the discharge of materials increases beyond the proper value for each screw. One of the conclusions (3) above has been theoretically analyzed, and a method to select the optimum dimensions of the screw and the profitable operating conditions of the conveyor has been obtained.