PURPOSE: To realize multistage gradation expression by providing a parallel planar lens between a photosensitive body and an LED head provided so that it faces to the photosensitive body and using a lens which has a refraction ratio distribution where the refraction ratio is decreased from center toward an outer edge part along with the moving direction of the photosensitive body.

CONSTITUTION: A refraction ratio distribution type lens (a Selfoc lens) 3 and a one-way refraction ratio distribution type slab lens 4 as the parallel planar lens are disposed each in parallel to the axis direction of the photosensitive body 1 between the LED head 2 and a photosensitive body drum 1. When the emitting light from the LED head 2 passes the lens 4, the light is refracted according to the refraction ratio distribution where the refraction ratio is decreased from the center toward the outer edge part along with the moving direction of the photosensitive body 1, and the refraction is not changed in the axis direction of the photosensitive body 1. Therefore, circular beam shape is changed into elliptical beam shape the minor axis direction which is same as the moving direction of the photosensitive body 1. And besides, since the axis length of the ellipse is changed by changing LED conducting time, the area of a dot is changed and gradation at every dot is safely performed.