A developing device adjoins a rotatably arranged a photoreceptor, and is internally provided with a rotatably disposed developing roller confronting the photoreceptor, a cylindrically formed flexible film member having a peripheral length longer than that of the developing roller and loosely mounted thereover, a first member for forming a slack of the film member, for example, a regulating blade or a cleaning blade, a second member for controlling the position of the slack of the film member, for example, a magnet.