PURPOSE: To obtain a wattmeter of simple constitution with high measurement precision by using a multiplier which performs digital arithmetic.

CONSTITUTION: An applied voltage signal (e) from an input terminal is insulated by a transformer PT and applied to and converted by the 2nd A/D converter 42 into a digital signal De. At this time, two converters 41 and 42 operate synchronizing with the clock signal output of a timing circuit 7. The digital signal Di from the converter 41 is applied to memory 5 and then applied to the input terminal X of a digital multiplying device 20 a specific time later. For the purpose, the number of stages of the memory 5 is selected properly and the phase of the output signal of the memory 5 is connected to the phase of the signal from the converter 42 to apply currents and voltage signals of the same time to input terminals X and Y. The device 20 multiplies the applied signals to the terminals X and Y by each other and applies the resulting signal to a mean value arithmetic device 30. Then, the averaging arithmetic is carried out with such an equivalent time constant that the basic wave or higher frequency component of the input signal is disregarded, and mean electric power is displayed on a display device 6.