PURPOSE: To miniaturize a lens and to reduce the quantity of an eccentricity so as to hold by cutting off a lens and its holding frame in a horizontal direction within at least an open angle 180° on an upper side which is the longitudinal direction of a slit.

CONSTITUTION: In case of considering the miniaturization of a slit scanning optical system, the sizes of the respective optical members constituting the optical system and the width of image formation luminous flux should be considered. In case of the slit scanning, the width direction of a slit of the effective range of the lens is decided from the mechanical width of the slit. Then the lens 22 having a diameter d1 is horizontally cut off only on the upper side from the optical axis of the lens. If the lens is assembled with the side where peripheral part remains by a lens barrel 21 having a diameter d2 while adjusting the focus of the lens at the time of assembling the lens, it can be settled within the maximum quantity of eccentricity in case of a round lens at most. Thus, the lens can be miniaturized and also it can be held without causing the error of eccentricity.