A photosensitive body has a photosensitive layer. An optical scanning
device has a deflector deflecting a light flux emitted from a light
source, and scans the surface of the photosensitive body by the
thus-deflected light flux. A dot is formed at a center between adjacent
light fluxes as a result of the adjacent light fluxes being overlapped
with one another in a sub-scan direction. A ratio of a static beam-spot
diameter Ws in the sub-scan direction on the surface of the photosensitive
body defined by 1/e.sup.2 of the maximum value in the exposure
distribution of the beam spot to an interval L between adjacent scan lines
satisfies the following formula: 1.2
