PURPOSE: To prevent errors due to an input containing DC and high frequency components and due to a frequency fluctuation by sampling a synchronous input signal and measurement input signal during one cycle of the synchronous input signal taken out from a cable line, at a constant interval.

CONSTITUTION: The synchronous input signal and the measurement input signals are inputted to an A/D conversion part 2 through transformers in an auxiliary detecting part 1. In the conversion part 2, an exact one cycle period is measured by making the synchronous input signal to pass through a BPF and the high frequencies higher than objective one for the measurement are cut by making the measurement input signals to pass through LPFs. Each input signal, after subjected to sample and hold by respective sampling hold circuits S/H, are summarized by a multiplexer MPX and subjected to A/D conversion then outputted to a processor part 3. At this time, a Fourier transformation using sampling data of one cycle is performed for the high frequency measurement, and a high speed sampling is made for realizing the accurate measurement. Finally in the processor part 3, a calculation of fundamental components is performed by Fourier transform means by using the data subjected to A/D conversion.