PURPOSE: To increase apparent resolution, by using the voltage obtained by peak-holding an output signal of an encoder, as reference voltage in case of A/D conversion, and executing an A/D converting operation with respect to this output signal.

CONSTITUTION: A reference voltage generating circuit PHC holds a peak value of an output signal St of an operation amplifier OA1 for amplifying and outputting a signal St generated by an encoder TPG, and applies the voltage corresponding to the peak value of the signal St, as reference voltage VREF, to a reference voltage input terminal 1 of an A/D converter ADC. In this way, reference voltage VREF used by the ADC is generated in the PHC, as that of a voltage value corresponding to the peak value of the output signal St from the TPG, therefore, even if amplitude of the output signal St of the TPG is varied by some causes, comparison voltages E1∼E4 applied to comparator COMP1∼ COMP4 in the ADC are varied, too, in accordance with variation of amplitude of the signal St, and therefore, even if the amplitude of the signal St is varied due to temperature change, the aging, etc., resolution is not varied by its variation.