To highly accurately detect the minute displacement of a target object to be detected after eliminating the effect of vibration transmitted from the point of installation.SOLUTION: An ultrasonic wave which is a transmission wave is sent to a target object and a reference object from a speaker 101 of a sensor unit 10, and its reflected wave is received by a microphone 102. A demodulated wave processing unit 110 cuts out, by a common time window, partial waveform data that corresponds to the reference object from reflected wave data derived by demodulating the received reflected wave and obtains phase characteristics by frequency analyzing the cut-out data. Then the demodulated wave processing unit 110 corrects the frequency characteristics of the reflected wave data at other reception times from a difference in phase characteristics between the partial waveform data at a reference time and the partial waveform data at other reception times, finds corrected reflected wave data from the frequency characteristics of the reflected wave data after the correction, and calculates the displacement information of the target object from a temporal change of the corrected reflected wave data for a prescribed past portion from the current time.SELECTED DRAWING: Figure 2【課題】設置箇所から伝わる振動の影響を排除して検出したい対象物体の微小な変位を高精度に検出すること。【解決手段】センサユニット１０のスピーカー１０１から対象物体及び参照物体に送信波である超音波を送波し、その反射波をマイク１０２で受波する。復調波処理部１１０は、受波した反射波を復調した反射波データから参照物体に対応する部分波形データを共通の時間窓で切り出して周波数解析して位相特性を得る。そして、復調波処理部１１０は、基準時刻の部分波形データの位相特性と他の受信時刻の部分波形データの位相特性との差から、他の受信時刻の反射波データの周波数特性を補正し、補正後の反射波データの周波数特性から補正反射波データを求め、現時刻から過去所定数分の補正反射波データの時間変化から対象物体の変位情報を算出する。【選択図】図２
