An average current value that is the average value of current values is found by connecting a detection resistor in parallel to two terminals that are the object of measurement, measuring the current value that flows in the detection resistor, and subjecting the current value to time-quadrature, and further, dividing the current value that follows integration by the integration time. The average current value is converted to a digital signal and sent to an arithmetic means (processor). The arithmetic means calculates the voltage value across the two terminals from the average current value and the resistance value of the detection resistor.