PURPOSE: To improve the accuracy of detection by determining the central position of detected light images from the state of the binary signal train obtd. by magnifying the light images, detecting the one-dimensional space position thereof and discriminating the level of the detection signals.

CONSTITUTION: A light image 11 is projected through a magnifying optical system 12 and is projected to a two-dimensional photoelectric detector 15. The image 11 is of a curciform shape, and the detector 15 has photoelectric arrays 25 disposed at 45° angle with respect to the X-, Y-axes of the image 11 on the surface to the projected thereof. The outputs from the detector 15, that is, the outputs of the respective photoelectric elements of the arrays 25 are inputted to a level discriminator 16 in accordance with the input from a sweep pulse generator 18. The discriminator 16 discriminates the levels of the respective inputs and forms a binary signal train. The outputs of the discriminator 16 are inputted to a signal processor 17, and the signals are processed with the processor 17.