The inventive device reduces noises and stabilizes the image quality of an image forming apparatus by reducing vibration thereof by reducing vibration of a driving source itself within a preset frequency range without changing the resonating condition of a housing. A mass member is added to a stationary shaft of a driving motor for rotating a polygon mirror. It then becomes possible to reduce the vibration and noise of an image forming apparatus by avoiding resonation by moving a resonance point by adding the mass member to the stationary shaft. Thus, a high image quality image forming apparatus can be realized.