An A/D converter performs successive A/D conversion operations that are synchronized with respective periods of an externally supplied clock signal. A set of output digital data produced from the A/D converter, following each A/D conversion, is acquired a plurality of times in succession within an interval that extends to the start of the next A/D conversion operation. If identical sets of data are not obtained in the successive acquisitions, then it is determined that there is failure of the A/D converter due to loss of the external clock signal.