A distributor 103 distributes an input signal into two lines of distributed signals with equal amplitudes and equal phases. Buffers 104, 105 suppress interference between distributed signals distributed into the two lines. A filter 106 performs a frequency selection which allows only a distributed signal in a predetermined band to pass. A differential amplifier 107 outputs a difference in amplitude components between a distributed signal frequency-selected by the filter 106 and the distributed signal which is not frequency-selected. This allows an attenuation characteristic or passage characteristic in a high-frequency band to be maintained.