To cope with temperature rises of the paper non-passage parts of, in particular, an ion conduction system intermediate body and a transfer roller during transfer and fixing that are carried out simultaneously.

Before a large-size job after the passage of a small-size paper, idle rotation is carried out as a down-sequence. A transfer current is detected at predetermined time intervals and temperature changes of the members are estimated from the amount of current change. The job is started after the value has stabilized.