in the light path between the cylindrical mirror 20 and the photoconductor 30 rectifies an error of image inclination of the polygon mirror 10 and at the same time lessens curvature of the field in other words the luminous flux is rectified in the direction perpendicular to the main scanning direction to a level image plane in the vicinity of a light collecting point if there occurs a tilt error on each reflective facet of the polygon mirror 10 a scanning line on the photoconductor 30 slips in the direction of sub scanning and irregular pitch is produced on an image the error by the inclination of image can be rectified by setting each reflective facet of the polygon mirror 10 and the light collecting surface of the photoconductor 30 in a conjugate relation produced by an optical system in the section perpendicular to the deflection plane by the polygon mirror 10 in the present invention a luminous flux is collected onto the polygon mirror 10 by the cylindrical lens 7 while each reflective facet of the polygon mirror 10 and the surface of the photoconductor 30 are arranged to maintain the conjugate relation by the cylindrical lens 25 in this embodiment it is further arranged to rectify an emitting luminous flux to a convergent luminous flux by the collimator lens 6 so that the curvature of the field in the vicinity of the photoconductor 30 can be rectified in other words when a convergent luminous flux enters a polygon mirror 10 the same as those of other rotative deflection devices the light collecting point on the polygon mirror 10 after reflection becomes almost a circular arc centering on the point of reflection and forms a curvature of the field if there is no.