A method of voltage measurement, and an apparatus for the same, having an improved accuracy and low cost is provided. The apparatus includes: a charging unit for charging a capacitor; a measuring unit for measuring a charge of the capacitor; and a microcomputer for calculating a voltage of the voltage supply. The microcomputer includes a correction value calculating program. The correction value calculating program applies a voltage of a correction measurement voltage supply to the measuring unit, measures the voltage of the correction measurement voltage supply, and calculates a correction value from a difference between a measurement value and a theoretical value of the voltage of the correction measurement voltage supply. The microcomputer also includes a voltage measurement program for subtracting the correction value from the measurement value of the charge during the prescribed period of time of charge measurement and for calculating the voltage of the voltage supply.