The present invention is to provide a method and apparatus of voltage measurement with a high resolution. An apparatus of voltage measurement includes: a buffer circuit for separating a voltage between terminals of a capacitor or an equivalent voltage into a first voltage region corresponding to a full scale of an analog/digital (A/D) converter and a second voltage region corresponding to a voltage exceeding the full scale of the A/D converter when the voltage between the terminals of the capacitor charged or the equivalent voltage is higher than the full scale of the A/D converter; the A/D converters corresponding to the first and second voltage regions for converting the voltages of the respective regions from analog to digital values; and a voltage measurement device for evaluating a measured voltage value from the digital values and calculating the voltage of the voltage supply.