A belt unit having a belt stretched by a plurality of rollers is removably mounted in an image forming apparatus. A pressure removing member moves a tension roller for applying tension to the belt from a first position to apply the tension to the belt to a second position to remove the tension from the belt. After the belt unit is positioned and mounted in the apparatus body by engaging the rollers with positioning portions in an apparatus body frame in a state in which the tension roller is moved to the second position by the pressure removing member, the tension is applied to the belt by moving the tension roller to the first position by the pressure removing member.