PURPOSE: To prevent the possibility of malfunction in controlling the microcomputer diagnosis of a copying machine by stopping the decision operation of a deciding circuit early in proportion to the change in output after turning off of a power switch.

CONSTITUTION: When a thermistor TH1 is disconnected, the output of a comparator Q3 goes to an L level, and a CPU1 of which Q3 is input-connected to PI2 judges said level and outputs a thermistor disconnection signal PO2 to turn a transistor Q7 on, thereby interrupting the conduction of electricity to a fixing heater H2 like in jamming. An alarm is given with a display device, and copying operation is interrupted to inhibit restarting of copying by turning on of a copying key. When a power switch is turned on, the electric power source of a circuit including a bridge and a comparator for detecting disconnection is connected not to ordinary VAA (24V) but to a power source VCC (10V) for a microcomputer in order to prevent the generation of malfunctions for alarming and inhibition of copying with a spurious disconnection signal. Since the outputting of the spurious signal is prevented by the above-mentioned method, the detection of the spurious signal with the CPU until the input to the PI1 disappears and the consequent malfunction are obviated. Similarly, the malfunction when the power switch is turned on is prevented.