PURPOSE: To attain the attenuation characteristic having a high attenuation capability even against low-order harmonics near the 2nd harmonic by combining plural digital filters such as difference filters or addition filters for extracting the fundamental wave.

CONSTITUTION: A difference filter eliminating the 4th, 8th, 12th and 16th harmonics with respect to the frequency of fundamental wave extracted from the system, an addition filter eliminating the 3rd, 9th and 15th harmonics, and a filter being the series connection of two-stage of addition filters eliminating the 2nd, 6th, 10th, 14th and 18th harmonics are combined. Thus, the attenuation capability for low-order harmonics near the 2nd harmonic is improved remarkably and the titled filter is applied to a digital protection relay installed to an ultrahigh and super-ultrahigh voltage system to eliminate harmonics surely and an excellent frequency characteristic is obtained by combining analog filters properly.