A voltage generated by a variable analog signal source is input to first and second input terminals of a multi-channel A/D converter via an analog signal input circuit including an analog switch and first and second amplifiers, and a resulting digital conversion value is written to a data memory via a microprocessor. Digital conversion values corresponding to voltages at the first and second input terminals that are obtained when the analog switch is opened are stored as first and second error voltages and used for producing first and second correction voltages, respectively. When the input voltage is low, a value obtained by dividing, by a compensation gain, a second correction voltage corresponding to a second input voltage that is produced by the second amplifier and input to the second input terminal as an enlarged range input terminal is selected.