PURPOSE: To measure a deviation between the optical axes between a light emitting part and a light receiving part by displacing a scanning beam so as to obtain a reference pattern having a curve in two-axial directions and projected onto a light receiving element, and by measuring a waveform picked up by the light receiving element.

CONSTITUTION: A scanning beam is displaced by shifting a photoelectric converting element 7 from a focal point of a light receiving lens 6, by inclining the light receiving lens 6 or by using aberration of the lens 6. Further, a pattern (reference pattern) such as an S-like curve which having curves in two axial directions is formed on the element 7. A waveform of the received beam with which the optical axes of light emitting and receiving parts 4 and 6 coincide with each other, has a predetermined lag with respect to a synchronized signal and a predetermined width. Meanwhile, if the optical axes are deviated from each other in either of the direction, the position of the receiving pattern is displaced, and accordingly, the waveform of the received beam varies. The received light is compared with a predetermined level (reference waveform), and a time relation between an output obtained thereby and the synchronized signal 12 is obtained. Accordingly, a non-received part of the scanning beam projected onto the element 7 in an S-like shape is determined, thereby it is possible to determine a deviation between the optical axes and a direction thereof.