springs 39 as shown in fig5 b therefore the turnback mirror 34 itself becomes liable to vibrate outwardly of the surface thereof with its opposite ends supported and therefore this vibration is detected by the sensor 3 attached to the back of the mirror and the amplitude value thereof is calculated by the calculation part 9 and a feedback gain is given to the piezoelectric element 1 attached to the back of the mirror like the sensor so that the amount of vibration may become minimum in the control part 10 it is desirable that the sensor 3 and the piezoelectric element 1 be attached and fixed to the mirror as in the embodiment in the optical scanning system the piezoelectric element 1 which has received a command gives vibration to the surface of the mirror in the form of returning an opposite phase to the mirror and continues these series of operations until the amplitude value becomes minimum thereafter developing powder toner is electrostatically attracted to the surface of the photosensitive drum 38 by a developing process not showing to thereby form an image of developing powder which is then transferred to transferring paper whereafter heat and pressure are applied thereto to thereby fuse the developing powder on the transferring paper while in the above described embodiment description has been made of a case where the piezoelectric element 1 is stuck on the turnback mirror 3 as an optical part the piezoelectric element 1 can be stuck not only on the turnback mirror 34 but on the vibrated portion of.