PURPOSE: To obtain an output corresponding to an effective value, by producing a square output signal obtained by squaring each instantaneous value of an input alternating signal to perform DC reproduction by making the square output signal pass through a clamping circuit and then carrying out the filtering of a low band and a pass band.

CONSTITUTION: An input alternating signal 1 (Asinθ) is squared by the square characteristics of FET8 and 9 of a squaring circuit part 2 and then added through an analog adding circuit 15. Thus a DC component (A2/2).cos2θ is obtained through a coupled capacitor 16. This signal is made to pass through a clamping circuit 3 to obtain a signal 7 of K.(A2/2).(cos2θ+1). Then a DC signal 5 is obtained through a low-pass filter 4. The signal 5 is equal to K.A2/2 to correspond to an effective value (A2/2)1/2.