PURPOSE: To prevent a shading phenomenon by minimizing variations in the reflectivity of a reflecting mirror accompanying variations in angle of incidence, by setting the polarization oscillation direction of a linear polarizing laser approximately at 45° to the rotary axis of a rotary polygon mirror.

CONSTITUTION: With regard to S-polarized light oscillating in a plane parallel to the axis of rotation of a rotary polygon mirror (b) and P-polarized light oscillating in a plane perpendicular to the axis of rotation, the reflectivity of the rotary polygon mirror having a protective film of an SiO on a vapor-deposited surface of Al increases or decreases as the angle of incidence of linear polarized laser light from a gas laser tube (a) in a polarization oscillation direction increases. The reflectivity of the rotary polygon mirror (b) to (p+s)-polarized light oscillating in a plane with 45° to the axis of rotation, however, has a constant value, without reference to the angle of incidence, which is the intermediate value of the S-polarized light and P- polarized light.