PURPOSE: To meter a watt stably without any influence from a load by using the logic of a comparator circuit and the duty cycle signals of a pulse width modulating circuit.

CONSTITUTION: In response to voltage signals eV applied, a pulse width modulatin circuit 11 performs the pulse width modulation thereby to generate duty cycle signals D having a pulse width. On the other hand,switches S1 and S2 are turned on and off so that the current signals, which are proportional to the consumed current appearing in the seconadry winding of a current transformer 15, are integrated by an integrating circuit 16. The resultant output is fed to a comparator circuit 17 so that the switches S1 and S2 are turned on and off by using the logic signals of the comparator circuit 17 and the duty cycle signals D of the pulse width modulation circuit 11. The time, for which the output voltage of the integrating circuit 16 reaches a preset level, is proportional to the product of the duty cycle D and the current signals i so that the frequency signals f proportional to the power can be obtained.