A current flowing through a measuring target coil increases when a pulse signal is high. A voltage corresponding to instantaneous value of the current through the coil is input to input terminal. A blanking signal generator circuit generates signal which has predetermined pulse width and rises in synchronization with rise of the pulse signal. During period in which this signal is high, first switch becomes ON and first capacitor holds the voltage at the input terminal. A second switch becomes ON when first switch becomes OFF to connect second capacitor to connection node between resistors, and disconnects second capacitor from connection node when the pulse signal becomes low. Second capacitor is charged with voltage corresponding to average value of an amount of increase in the current through the coil. A voltage obtained by adding the voltage charged in first capacitor to this charged voltage is supplied to third capacitor.