PURPOSE: To relieve an image forming device operator from psychological unpleasantness in such a manner as to enable him (or her) to tell length of time he (or she) has to wait until heating is completed, by making it possible to indicate variation of temperature of a heating body, such as a fixing equipment, etc., during the heating period.

CONSTITUTION: A temperature sensor 2 having detected temperature of a fixing equipment 1 converts the detected temperature into voltage and puts it into a comparison circuit 3. Whenever the input voltage become higher than a voltage corresponding to the preset temperature, the comparison circuit 3 impresses voltage of respectively different value into one of liquid crystal indicating circuits 9W13 through one of output circuits 4W8. It is possible to tell from indication colors of the liquid crystal indicating circuits 9W13 a length of waiting time until completion of heating.