PURPOSE: To automatically measure a harmonic distortion factor corresponding to different amplitude of a reference alternating signal, by applying a sinusoidal wave being an input application signal to a digital communication equipment after amplitude modulation with a low frequency signal.

CONSTITUTION: An output of a triangle wave oscillator 21 is applied to an amplitude modulator 23 via a resistance attenuator 22, and an output of a sinusoidal wave oscillator 1 is amplitude-modulated with the output of the triangle wave oscillator 23. The output of the modulator 23 is inputted to a system 4 to be measured via a resistance attenuator 2 and a low-pass filter 3, and the output of the system 4 is applied to a band-pass filter 6 selecting harmonics and a band-pass filter 5 selecting the applied frequency. The output of a differential amplifier 11 to which the output of the filter 5 and 6 are inputted via detectors 7 and 8 and logarithmic converting amplifiers 9 and 10 is inputted to a peak detector 25. The peak detector 25 is reset at each one period of the triangle wave with a detector 24 and the worst value of the distortion factor can be displayed on a level indicator 12.