PURPOSE: To release temporary stop of a microprocessor (MP) when an A/D converter to be tested is defective by adding a timer circuit.

CONSTITUTION: When the A/D converter A is defective and a conversion end signal AEO is not active and an AND output LO of (dotted line part of waveform 2) 1 and 2 is inputted to a hold input H1 of a microprocessor D, the processor D keeps the hold state. A timer circuit B is operated simultaneously with an output STO of a conversion start signal generating circiut S is active. The time setting is made longer to a time when the conversion end output AEO is made active. Even if the conversion end signal is not active, an output TO of the timer B is made active, and the conversion end signal and a timer output signal are inputted to an AND circuit C, the AND output LO goes to a waveform 5 as a resust, and even if the converter A is defective and no conversion end signal is outputted, the hold state of the processor D is released.