An apparatus for forming a plurality of different color visual images on a photosensitive
member is provided. A transfer medium driven by a driving roller and receives the
plurality of different color visual images at a transfer section from the photosensitive
member. The transfer medium superimposes and transfers the different color visual
images to a transfer sheet a plurality of mark sets each formed from a set of different
color marks (Bk, Y, M, C) aligned in a movement direction is formed on the transfer
medium. Respective marks are detected by a sensor. An average of displacements
of respective different color marks from a reference position is then calculated.
The sensor is positioned being distanced from the transfer section by a prescribed
length. The prescribed length is calculated by multiplying a conveyance length
the transfer medium travels when the driving roller rotates once by an integer number.
