PURPOSE: To smiplify the circuit constitution of the device for detecting the light spot positions, by performing logarithmic processing of two groups of photoelectric currents which are outputted from semiconductor position detecting elements.

CONSTITUTION: One input is applied to operational amplifiers A1 and A2 from a common terminal C of the semiconductor position detecting elements. The other inputs are applied to said amplifiers from first and second terminals A and B of the semiconductor position detecting elements. Logarithmic compression diodes D1 and D2 are connected to the operational amplifiers A1 and A2. The outputs of these amplifiers A1 and A2 are processed in a differential amplifier A3, and the ratio of the two photoelectric currents which are outputted from the semiconductor detecting elements corresponding to the positions of the light spot is determined. By the method of the logarithmic processing, the circuit constitution of the device for detecting the light spot positions is simplified.