In FIG. 34: indicated at ???A is a region where a developing unevenness is liable to occur; indicated at ???B is a region where the effect of the a.c. component does not appear; indicated at ???C is a region where the toners are liable to return, i.e., where the color mixing is liable to occur; and indicated at ???D and ???E are regions where the effect of the a.c. component appears so that no col