Abstract: In measuring analog input voltages through isolation transformers, an analog input voltage V.sub.in is applied across a primary winding of the isolation transformer, voltages V.sub.A and V.sub.B developing across the secondary winding of lapses of time T.sub.A and T.sub.B (T.sub.A < T.sub.B) from application of the analog input voltage are detected, and by using measured times T.sub.A and T.sub.B and secondary winding voltages V.sub.A and V.sub.B, the analog input voltage V.sub.in is calculated in accordance with an expression ##EQU1## whereby influence of sag due to the isolation transformer is compensated for.