PURPOSE: To detect spotted position of light with high accuracy, by compensating the output variation of the light source and the variation of the transmitted quantity of light of the light guide.

CONSTITUTION: The output of a laser oscillator 1 is divided into three parts, a detecting beams 2a for variation of quantity of light, a picture producing beam 2b, and a position detecting beam 2c, by a beam splitter 2. The detecting beam 2a for variation of the quantity of light is received by a photodiode 3 and added to the input terminal of a position detecting circuit 14. On the other hand, the picture producing beam 2b and the position detecting beam 2c are scanned by a polyhedral mirror 6 driven by a motor 7, and the picture producing beam 2b is bent by a folded mirror 8 and falls on a picture producing table 15. The position detecting beam 2c passes through a glass scale 9 in which lines of a fixed pitch are arranged and generates an impulsive signal. The passed light enters a photomultiplier 11 after it is condensed by a light guide 10 and is added to the position detecting circuit 14 after it is photoelectrically converted.