PURPOSE: To curtail the preparatory time, facilitate accurate temperature control, and increase thermal efficiency in a small size, by using the heat plate itself as the heating element and minimizing the heat capacity.

CONSTITUTION: A heating element 19 is a thin rectangular piece made of metal material having a high specific resistance. A plurality of such heating elements 19 are laid on a substrate 18 made of heat resistant material having a low thermal conductivity along the moving direction of a paper 7, and are connected to electrodes 20, 21 at the ends. A reference voltage corresponding to the desired temperature is applied to a terminal 24, and is applied to the heating elements 19 after amplifying the power by an amplifier 23. An additional resistor 25 having a lower resistance than the heating elements 19 is connected in series thereto. By the voltage proportional to the current generated in the terminal 26 and the voltage generated in the terminal 7 to act on the heating elements 19, the resistance of the heating elements 19 is detected in a resistance detecting circuit 22. The resistance of the heating elements 19 varies with the temperature. Therefore, once the resistance is detected, the temperature can be known. Hence, by negatively feeding back the voltage corresponding to the resistance to the reference voltage (temperature) applied to the terminal 24, the heating elements 19 may be held at a constant temperature.