PURPOSE: To correct the reference voltage with a value memorized after being obtained by a method wherein a stable standard voltage is compared with the reference voltage as input voltage while fluctuation of the battery voltage (reference voltage) is negligible in substance and the results are converted to digital system.

CONSTITUTION: When a switch S1 is turned ON only for a certain period T1 with a control computation circuit 8, an input voltage Ei is integrated in the direction opposite to Ei with R, C1 and an operation amplifier 6. When a switch S1 is turned OFF and S3 is turned ON with the control circuit 8, a power source voltage VSS opposite in the polarity to Ei is integrated with C1 and as the output E0 decreases to O, the output of a comparator 7 inverts. As soon as the control circuit 8 turns OFF the switch S3 detecting the inversion, the period T2 during which the switch S3 is ON is converted to a digital value P0. The period T2 is proportional to the input voltage Ei and hence, when stable clocks are counted only during the period T2, the input voltage Ei is converted accurately to a correct digital value P0. Thus, the battery voltage can be ultilized for an electromagnetic flowmeter or the like using a battery for the power source as reference voltage.