Abstract: A reference voltage for an analog-to-digital (A/D) converting unit is converted into plural digital values through the effect of the A/D converting unit. A difference between an average value of the digital values and an ideal reference value is derived. If the different is larger than a predetermined allowable value, it is determined that a transient abnormality takes place in an A/D converter system. The last measured reference voltage and its related digital input signal are disallowed to be used as measured values. If detection of a transient abnormality serially takes place predetermined times, it is determined that a constant abnormality takes place in the A/D converter system.