PURPOSE: To suppress longitudinal line distortion and color slurring by vibrating a mirror with simple structure.

CONSTITUTION: An original on a platen glass 1 is scanned and exposed by an exposure light source 2 of an optical system and the optical image of the original to be obtained is led to an image pickup unit 9 by plural mirrors 2, 5, 6 with different natural frequency. When the natural frequency of each mirror is made different by changing the thickness of the 2nd and 3rd mirrors, even when one mirror is resonated, since the resonance of the other mirror is not caused, the vibration and amplitude of the entire mirror are suppressed small and the longitudinal distortion around both ends in the main scanning direction is decreased. Moreover, the thickness of the 1st mirror 2 is increased more than the width of the reflecting face in the subscanning direction to enhance the rigidity in the broadwise direction, then the vibration in that direction is suppressed, the deviation in the read position around the center in the main scanning direction is decreased and color slurring in the subscanning direction is suppressed.