PURPOSE: To provide a centroid detector optimum for a fuzzy inference device by hardware to execute high-speed processing by coupling a light source array and a photo-position detector with a glass substrate.

CONSTITUTION: A light source control circuit 13 and an LD array 21 are respectively arranged as chips on one sheet of board 41. The glass substrate 25 is tightly adhered to the upper side end face of the LD array 21. Chips of an photo-position detector 23, a power source 27 for photo-position detection and a signal processing circuit 26 are arranged on a board 42 facing the board 41. The boards 41, 42 arranged with the respective chips are combined with each other so that the photodetector 23 and the glass substrate 25 are brought into tight contact with each other. The reason for using the LD array 21 as the light source array lies in that the photo-position detector 23 merely faces the light source array via the glass substrate 25 without via a lens system, etc., and that the divergence angle of exit light is small and the arraying is actually possible as the light source element, in order to prevent crosstalks.