A mixer shifts the center frequency of a received signal to a baseband according to a center frequency detected by a center frequency detector or a center frequency estimated from ship speed, for example, by a center frequency estimator. The passband of a variable passband filter varies according to a bandwidth detected directly from the received signal by a bandwidth detector or a bandwidth estimated from the pulselength of a transmitting signal by a bandwidth estimator, whereby the bandwidth of a baseband signal output from the mixer is limited to remove undesired noise components in the received signal.