Under normal circumstances, the electrical power supply has a frequency of 50 Hz or 60 Hz representing a single sine wave. However, due to a pulsating circuit load of a device, an actual current includes large harmonic components and does not represent a sine wave. This harmonic distortion flows into the supply and distribution systems and may cause disorder such as abnormalities on other connected equipment or distortion of alternating voltage. The FFT analyzer enables analysis of the harmonic distortion. By measuring variation of power current with a clamp type ammeter and performing frequency analysis of power current waveform with the FFT analyzer, harmonic distortion of higher order component and total harmonic distortion of the fundamental wave (50 Hz or 60 Hz) can be evaluated.