To calculate total harmonic distorsion by generating a digital signal showing harmonic signal after obtaining a difference between the measured sample value of waveform and the calculated value of basic frequency element and dividing the signal value by the basic frequency element value.

A distribution system 3 is provided with phase conductors 5A to 5C, neutral conductor 5N and ground conductor 5G. Current transformers 7A to 7C, 7N and 7G detect current flowing in a conductor and transformers 9A to 9C detect voltage between phase and neutral conductors, further a transformer 9N obtains voltage between neutral and ground conductors. A ranging circuit 11 converts a current signal and voltage signal of -10 to 0 volt into a signal of +10 volt, and it is subject to A/D conversion 13, then the result is inputted into a digital processor 15. A converter 13 samples analog voltage and current at a sampling rate that is determined by the interruption signal generating in the processor 15. Then, a harmonic signal is generated as the difference between the sample value and basic frequency element of the sample, and an output signal is generated as a ratio of the RMS value of harmonic signal to that of basic frequency signal.