PURPOSE: To perform position detection accurately in a short time by reading the contents of a brightness memory within a specific region from the center point of a visual field, and deciding on the coordinate position of a target point from an address position corresponding to signals with logic "1" among the read signals.

CONSTITUTION: A computer 5 executes a graph generating program to decide on position information on a graphic form, and then writes brightness information in the brightness memory 11. A scanning mechanism 12 reads the brightness information out of the brightness memory 11 synchronously with the scanning of a graphic display device 1 and displays it on the graphic display device 1. The intersection of tracking lines 7 and 8 is put in the visual field 9 of a light pen 2, and then aposition detecting mechanism 13 inputs the outer circumference of the visual field 9 and the information on the intersection of the lines 7 and 8 to the computer 5 to calculate the position coordinates (x0, y0) of the center point 11 of the visual field.