An image forming apparatus includes: image bearing members; a belt unit including an endless belt; a slackness forming unit for performing a first function of forming a belt slacking region, at least at a portion where the belt opposes the image bearing members, by causing the slackness forming unit to act on the belt unit and for performing a second function of eliminating the belt slacking region; and a guiding unit for guiding the belt unit so that the slackness forming unit performs the first function in a state in which the belt is spaced from the image bearing members and so that the slackness forming unit performs the second function in a state in which the belt slacking region contacts the image bearing members.