A test pattern P3 longer than a length Ld corresponding to a circumferential length of a developing roller is formed along a moving direction of a photosensitive member (or developing roller). The developing roller transports a great quantity of toner on its surface in a first revolution thereof and hence, the test pattern has a high density. When the residual toner runs low, the toner transport quantity is decreased in the subsequent revolution, so that the test pattern P3 is decreased in the density at its portion beyond the length Ld from its head. If a density difference is observed at place corresponding to a boundary between image portions formed in the first revolution and the second revolution, it is concluded that image quality is degraded due to the shortage of residual toner.