PURPOSE: To measure the ripples on the mirror surface of a polygon scanner and the deformation of the mirror surface by the rotating centrifugal force.

CONSTITUTION: A laser light source 1 is expanded with a beam expander 2. The laser beam L is cast on a scale 3 having a slit pattern (e). The laser beam emitted from the scale 3 is cast on a slit plate 10 through a first cylindrical lens 4, an Fθ lens 5, a polygon scanner 6, a second cylindrical lens 7 and a condenser lens 8 which are arranged in the reverse pattern from the constitution of a laser printer. The image of the pattern (e) of the scale 3 is focused on the slit plate 10. The pattern of the scale 3 Which is selected with a slit 10a of the slit plate 10 is cast into a photodiode 9. The signal which is affected by the state of a mirror surface 6a of the polygon scanner 6 is obtained from the photodiode 9.