PURPOSE: To distinguish the breakage of a heater from a failure due to constant turning ON/OFF of a control switch to be sensed by generating first and second inspection timing signals from a heater ON/OFF signal of a controlling computer.

CONSTITUTION: A heater ON/OFF signal (a) from a controlling computer is converted into a predetermined level in a computer interface circuit 6 to be on ON/OFF signal (b), which is sent to a timing circuit 7. The circuit 7 generates a first inspection timing signal (g) and a second inspection timing signal (h) and sends them to a judgement logic circuit 10. On the other hand, voltage (c) between switches is applied to a detector 8 for voltage between switches and voltage (e) between heaters is applied to a detector 9 for voltage between heaters respectively. The detectors 8, 9 generate voltage signals (d),(f) according to applied voltage respectively to output them to the circuit 10. The circuit 10 logically distinguishes heater breakage from an error in constant turning ON/OFF of a control switch and sends either of a heater signal (i) and switch signals (j),(k) to an output drive circuit 11.