PURPOSE: To obtain the effective value of an AC signal including a distorted wave relatively quickly and automatically by decomposing a signal to be measured into frequencies and performing the operation processing.

CONSTITUTION: A signal (a) to be measured and a sweep lamp signal (b) from a voltage control oscillator 2 are mixed in a mixer 1, and a pulse train output signal (c) having a frequency distribution similar to that of the signal (a) is outputted from a detector 14. An inverting amplifier 5 not only inverts the signal (c) but also outputs a signal (d) sifted by an offset voltage V. After a time tb, the output voltage of a crest value detecting circuit 61 is larger than the input voltage, and the output of a comparator 71 is in the high level, and a monostable multivibrator 81 is triggered, and the output of a crest value detecting circuit 62 is initialized by a signal (e). At a time tc, the output of a comparator 72 becomes high-level, and a monostable multivibrator 82 is triggered, and the output of the crest value detecting circuit 61 is initialized by a signal (g). An output signal (h) of the crest value detecting circuit 61 passes a multiplier 10, a multiplier 11,and an adder 12 to obtain a square root in a square root device 13.