In an image forming apparatus, an image forming unit configured to form a toner image on a transfer medium includes four sets of a photoconductor drum and a development roller, respectively, for forming toner images of first, second, third and fourth colors, arranged in this order, in a direction opposite to a direction of movement of the transfer medium moving toward a detector configured to detect a toner image formed on the transfer medium. A controller executes a bias-corrective test image forming process in which a first bias-corrective test image of the first color is arranged in a position downstream of other three first bias-corrective test images, and a second bias-corrective test image of the first color is arranged in a position between two first bias-corrective test images selected among other three first bias-corrective test images, in the direction of movement of the transfer medium.