In an image forming apparatus, a first photoconductor drum for use in at least in a monochrome printing mode is located upstream, in a direction of transport of a medium, of a plurality of second photoconductor drums for use in a color printing mode. A transfer bias for each of transfer members is regulated in such a manner that a transfer bias applied in the monochrome printing mode between a second photoconductor drum located adjacently downstream of the first photoconductor drum and the corresponding transfer member is larger than that applied in the color printing mode. A developing bias for each of the development rollers is regulated in such a manner that an absolute value of a developing bias voltage applied to a development roller corresponding to the first photoconductor drum in the monochrome printing mode is smaller than that applied in the color printing mode.