PURPOSE: To prevent the output of an integration voltage from being disturbed remarkably, or rised by providing an integration circuit, a fault supervisory circuit and a holding circuit holding an output signal.

CONSTITUTION: An output signal whose duty ratio is variable outputted from a processor 2 normally is converted into an analog voltage signal by the integration circuit 8. On the other hand, if a fault such as runaway takes place in the processor 2, the fault supervisory circuit 3 detects it to reset the processor 2 and to start a timer means 4. Thus, the integration output of the integration circuit 8 is held by holding circuits 5,6 during the time required when the processor 2 is reset til the state is restored normally. Thus, the integration output signal of the integration circuit 8 is fluctuated largely due to runaway of the processor 2 is prevented.