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.