d become bk 600 v m 530 v c 500 v and y 480 v and image distortion resulting from the discharge phenomenon may be prevented besides background fog which can be generated more easily by decrease in v d is made less conspicuous by selecting bk for which background fog is most noticeable as the first color and y for which the background fog is least noticeable as the last fourth color in the present embodiment m and c images may be formed in an arbitrary order in the foregoing embodiments weak exposure is executed for a color in the image area in which such color is not used however the color used in the image formation is used in various levels from a solid image to a pale halftone consequently in executing weak exposure for a color not used inb an image formation if color used in the image formation becomes thinner or more pastel such pale image may be detrimentally affected when background fog of a color not used in the image formation is elevated by weak exposure the present embodiment avoids such a drawback by selecting the maximum density largest light emission data for each dot of.