In an image forming apparatus which expresses lamp lighting energy
corresponding to input gray scale data on the basis of on/off of lighting
lamp of plural sub-dots corresponding to different lamp lighting times of
different assistant scanning positions, with respect to such image that
rapidly changes from a state close to white level toward a state close to
black level, or with respect to such image that rapidly changes from a
state close to black level toward a state close to white level, in order
to decrease that a thin white stripe or a thin black stripe happens on
the border portion, a random generator 34 and a adding circuit 33 are
provided so that: whenever the gray scale value is inputted to the gray
scale value input register 31, with respect to the lamp lighting energy,
a random number of predetermined level is added.