PURPOSE: To detect a straight line of a wafer, etc., by simple, small-sized constitution with high precision by moving a matrix image by specific algorithm and differentiating the light image input of the body to be detected.

CONSTITUTION: The image input obtained through an electric converting mechanism 12 by picking up an image of the wafer 4 as the body to be detected by a binocular microscope 8, CCD camera 9, etc., is stored in the video memory 13 of a control mechanism 14. Then, when the matrix image input to the memory 13 is moved in two orthogonal directions by the algorithm of a sober operator through a CPU, the image input is differentiated through a differentiating circuit 15 and stored in a differential memory 16. Its storage contents are sliced by a specific threshold value and further processed by a boundary detection processing circuit 23 and a straight-line detection processing circuit 22 to detect the beltlike area of the wafer 4 as a straight line. Consequently, the simple, small-sized constitution detect the straight line of the wafer, etc., with high precision by the simple algorithm without using the algorithm of a matching method, etc.