The decrease of the transfer efficiency of transferring a toner image from a photosensitive drum onto an intermediate transfer belt is suppressed by the structure of an image forming apparatus including: a photosensitive drum holding member that pivotally supports the photosensitive drum at both ends thereof; a transfer roller holding member that pivotally supports a primary transfer roller at both ends thereof; and a transfer roller holding unit that determines a relative position between the photosensitive drum holding member and the transfer roller holding member by guiding the transfer roller holding member in a direction substantially perpendicular to a running surface of the belt and abutting an opposing part of the photosensitive drum holding member and an opposing part of the transfer roller holding member with each other, the opposing parts opposing each other.