An image forming apparatus of the present invention includes an endless belt that travels circularly by a drive member and has an outer peripheral surface on which an image is formed; a tension applying member around which the endless belt is trained and that applies tension to the endless belt by pressing an inner peripheral surface of the endless belt; a detection member that detects the image formed on the outer peripheral surface of the endless belt; a support member to which the detection member is attached and that movably supports the detection member with respect to an apparatus main body; and a positioning member that positions the detection member with respect to the outer peripheral surface of the endless belt at a portion pressed by the tension applying member.