In the image forming apparatus of this invention, a bending habit is applied to a PET sheet by a heating iron to form an end mylar into an L shape. In a full-color mode, the end mylar presses an end of an intermediate transfer belt while being elastically deformed. In a black monochrome mode, the intermediate transfer belt is spaced from photosensitive drums by up to 2 mm compared with the full-color mode. Instead of the photosensitive drums spaced from the intermediate transfer belt, the end mylar regulates the end of the intermediate transfer belt in a vertical direction by a slightly small pressing force with a recovered bend, whereby flapping of the intermediate transfer belt is reduced.