PURPOSE: To discriminate square materials to be discriminated such as pieces of fried bean curd in a high speed for the purpose of sorting them, by extracting the size and the shape on a basis of the position of the center of gravity of sample points in every side of the picked-up image of the material to be discriminated and comparing extracted them with reference values.

CONSTITUTION: A square material U to be discriminated such as a piece of fried bean curd has the image picked up by a TV camera 1 in the course where it is transported by a conveyor, and coordinate values of respective plural sample points P on four sides are obtained from the picked-up image face by a picture processing device 2, and the position of the center of gravity of sample points P in every side is calculated. The size and the shape of the material U to be discriminated are extracted as data on a basis of lines connecting said positions Pa and Pb of centers of gravity in sides A and B facing to each other and connecting said positions Pc and Pd of centers of gravity in sides C and D facing to each other, and these data are compared with reference values to discriminate the material U. For example, valves of air nozzles N1 and N2 are controlled by a sorting device 3, which is operated in accordance with the output of the device 2, to sort the the material U to be discriminated such as a piece of fried bean curd in respect to size.