An image forming apparatus is described. The image forming apparatus may include a casing, developing agent carriers, an image carrier, and a transferring unit anteroposteriorly detachably mountable to the casing on a front side of the casing. The transferring unit includes a projection at a rear end portion thereof and the casing includes a restricting portion that is arranged lower than the projection of the transferring unit midway through mounting and is engaged with the projection at completion of the mounting of the transferring unit. At least one of the casing and the transferring unit includes a first guide portion for guiding the transferring unit by inclining the transferring unit so that the rear end portion of the transferring unit midway through the mounting is more downwardly inclined than the rear end portion at the completion of the mounting of the transferring unit.