PURPOSE: To achieve a higher measuring accuracy of the apparatus, by providing a light emitting diode and a constant voltage diode to be inserted into a voltage circuit to be measured and a differential amplifier.

CONSTITUTION: A bias power source voltage is divided with a light emitting diode 6 and a constant voltage diode 8 into voltages in a measuring range. The light emitting diode 6, an element which varies the intensity of emission light with an applied voltage, has an insulation against a high voltage and outputs an analog signal (a) generated by optical coupling to a differential amplifier 10. On the other hand, a light emitting diode 7 connected to a reference voltage 9 outputs an analog signal (b) equivalent to the reference voltage 9 to the differential amplifier 10, which compares the analog signals (a) and (b) and outputs a difference to a correction circuit 11. After being subjected to a signal correction with the correction circuit 11, a differential output from the differential amplifier 10 is converted into a digital signal with an A/D converter 13 and an indicator 14 to be displayed 12. This achieves a higher measuring accuracy of the apparatus.