A scale having marks disposed at a predetermined interval is provided on an intermediate transfer belt. A sensor detects the scale and outputs a binary signal. A counter counts a wave number of the binary signal. The wave number of the binary signal detected when the sensor detects a normal scale within a predetermined time is stored in a memory. A difference between the wave number stored and a wave number counted within a same period of time as the predetermined time is greater than a predetermined value, degradation of the scale and a change in a control of speed of the intermediate transfer belt into a control by a dummy signal is displayed on a display.