PURPOSE: To eliminate the unstableness of rotation and the generation of a noise due to the damage of a bearing part and to make a stable scan with a light beam even during high-speed rotation by fitting a rotary polygon mirror to a rotor part with requested strength without damaging the bearing part of a fixed shaft.

CONSTITUTION: The rotating device provided with a bracket 2 on which a stator 9 is fixed and provided, a pivot shaft 3 fixedly supported on the bracket 2, bearings 4a and 4b provided on the pivot shaft 3, and a rotor part 5 which is fitted outside rotatably through the bearing parts is fitted with the rotary polygon mirror 6 by using a fitting member 7 and a clamping member 8 and a through hole 13 is formed in the bracket surface which faces the clamping member 8 in the shaft direction; when rotary polygon mirror 6 is clamped to the rotor part 5 by using the clamping member 8, a supporting member is inserted into the through hole 13 to support the rotor part 5.