A signal processing device (2) includes an amplifier (3), an A/D converter (4) and a signal processor (5). The amplifier (3) amplifies a sensor signal that is supplied from a radio wave sensor (1) and depends on a motion of an object. The A/D converter (4) converts the sensor signal amplified by the amplifier (3) into a digital sensor signal. The signal processor (5) converts the sensor signal output from the A/D converter (4) into a frequency-domain sensor signal by Discrete Cosine Transform processing, estimates a background signal contained in the frequency-domain sensor signal, based on an error signal obtained from the sensor signal and a reference signal which is white noise, and determines a filter coefficient according to the estimated background signal, thereby operating as an adaptive filter for removing the background signal from the sensor signal in the frequency domain.