PURPOSE: To stop power supply to a heat source accurately in a short time if a heating roller rises in temperature abnormally by detecting set temperature optically and sending a signal if the measured temperature of a temperature measuring means arranged nearby the heat source reaches the set temperature.

CONSTITUTION: A rod-shaped glass thermometer 12 is fitted to the side plate 8 of a fixing unit and a liquid reservoir part 12b is arranged where the radiant head from a heater 6 is received directly. A photointerrupter 13 as a detecting means is arranged by the thermometer 12, the light emitted by an LED 13a is reflected by the surface of a liquid column part 12a, and its reflected light is photodetected by a photodetecting element 13b. This device has a difference in the reflected light quantity depending upon whether there is liquid at the reflection position of the liquid column part 12a or not, so the photodetection quantity also varies and whether the liquid level reaches the reflection position or not, namely, whether the set temperature is reached or not can be detected. Therefore, when an abnormal temperature rise state is entered as a result of continuous power supply, the liquid level of the thermometer 12 rises, which is detected by the photointerrupter 13; and its signal is sent to a CPU and the power supply to the heater 5 is stopped.