a moment that an input selection switch not shown is closed and with a subsequent gradual reduction in the voltage due to the influence of sag which gradual reduction continues until an undershoot is generated by opening the input selection switch at t o a specified value of time t and finally the secondary voltage approaches zero for accurate measurement of an input voltage v in voltages v a and v b are measured at optional time points t a and t b t a u2260 t b after the overshoot is terminated and a point at which an extension line connecting the voltage values v a and v b is crossed with the ordinate at t 0 is determined where symbol r represents a sum of direct current resistances of a primary winding of the transformer and the input selection switch l a primary inductance of the transformer and t the lapse of time from the closure of the input selection switch the relation between the secondary voltage v o of transformer and analog voltage v in is given by the following equation equ3 from equation 1 the voltages v a and v b at time points t a and t b are obtained as equ4.