PURPOSE: To keep mixing up ratio of undersize material to oversize material less than an aimed value without lowering yield ratio by changing supplying positions of granular and massive materials to a sieve according to a wear loss of the sieve and the conditions of the granular and massive materials so as to adjust the sieving surface area and screening the materials for separation.

CONSTITUTION: In screening granular and massive materials such as iron ores, the supplying positions of the granular and massive materials are changed according to the wear loss of a sieve and the conditions of the granular and massive materials so as to adjust the sieving surface area and then screening is carried out. For example, a shuttle reverse conveyor 1(2) movable toward the flow direction 6 of raw materials flow on a sieve 4 is installed and the conveyor is moved to the conveyor moving direction 11 so as to change the supplying position of the raw materials and attain a defined screening efficiency. As a result, without lowering the screening yield, mixing up ratio of undersize materials to oversize materials can be kept less than an aimed value and lowering of yield and quality of the materials is prevented.