A received signal obtained from an antenna is subjected to high-frequency amplification. The amplified signal is supplied to a first bandpass filter, which extracts only signals of all the channels of a communications system concerned while filtering out other radio signals. The extracted signals are frequency-converted by using a local oscillation frequency, and only a desired wave is passed by a second bandpass filter. The desired wave is supplied to a sample-and-hold circuit, which performs sampling according to the bandwidth-limiting sampling theorem. A resulting discrete signal is supplied to an I-axis-component and Q-axis-component separating circuits, where the polarity of sample values is inverted for every other clock pulse with respect to each of the I and Q axes to thereby effect Hilbert transform. Resulting two orthogonal components on a phase plane are supplied to a complex coefficient filter.