PURPOSE: To perform the accurate evaluation with respect to the rotary speed of a polygon mirror, by reading the speed variation of beam scanned at an arbitrary point in a scannable deflection angle.

CONSTITUTION: When a polygon mirror 10 is rotated to the direction shown by an arrow and laser beam 12 on one surface of the mirror scans within a range of a scannable deflection angle θ, the reflected laser beam is changed to a linear shape in the transmissivity thereof toward the direction shown by an arrow A and the transmission light amount is increased with the advance thereof to the arrow direction. The reflected beam is transmitted through a pervious glass plate 14 to be re-reflected as shown by an arrow 20 by a secondary reflecting mirror 16 and focused to a photoelectric element 18. In this case, beam is changed to a current by the photoelectric element 18 and voltage generated in the photoelectric element is changed corresponding to the change in a light amount transmitting through the pervious glass plate 14 and the rotary angle of the mirror. As mentioned above, the relation of the change in voltage generated in the photoelectric element 18 and the rotary angle of the mirror 10 is evaluated to enable the measurement of speed variation.