To prevent deformation of the reference surface of a fitting base for assembling a rotating polygon mirror.

The rotating polygon mirror 1 is pushed with an elastic mechanism 6 to a base surface 4c of the fitting base 4 and is coupled integrally with a rotor 5 of a motor. In the process for depositing a rotating shaft 2 which is supported to rotate with radial bearings 3a to an engaging hole 4a of the fitting base 4 by the methods such as shrinkage fit and pressurized insertion, an escaping route by a pair of large diametric portions 4e, 4f is provided at the upper and lower portions of the center portion 4d for tightening fit of an engaging hole 4a in order to prevent deformation of the fitting base 4.