In a sensor system for detecting a body motion based on a received wave of transmitted radio waves, the amplitude change of the received wave is detected and a loss of a frequency component indicative of one or both of heart rate and respiration is detected in the frequency analysis result of the received wave The body motion is detected.送信した電波の受信波に基づいて体動を検出するセンサシステムにおいて、前記受信波の振幅変化が検出され、かつ、前記受信波の周波数解析結果において心拍及び呼吸の一方又は双方を示す周波数成分の欠落が検出された場合に、体動を検出する。