To enable high-precision alignment to be performed by reducing a mutual difference in radius, to enable plural rotary polygon mirrors to be machined to be performed at the same time, and to improve the surface tilt and rotating operation precision when a rotary polygon mirror used for a laser printer is manufactured.

The rotary polygon mirror is provided with a structurally vertically penetrating inner peripheral surface and the outer peripheral a machining/ fitting jig shaft part which comes into contact with an inner peripheral part expands by tens of μm with oil pressure to achieve fixation machining.