PURPOSE: To certainly measure the position of light beam with a simple structure having durability, by scattering light beam in a box body and detecting scattered beam with two photoelectric position detectors in the periphery of the box body.

CONSTITUTION: Light beam 6 is allowed to be projected an to a light pillar 7 from the transparent incident surface side of a box body 1 containing a light scattering means 2 in which a transparent fine powder scattering beam is suspended. Two photoelectric position detectors 5 are provided at different positions in the periphery of the box body and scattered beam is condensed on the detectors 5 by an optical lenses 3 to issue electric signals. On the basis of the signals from two places, the incident position of the light beam is calculated corresponding to the direction of the light pillar 7. Therefore, the position of light beam can be measured certainly with simple constitution.