To discriminate an object among existing objects that can be assumed as being quiescent.SOLUTION: An ultrasonic wave which is a transmission wave is sent to a human body 12, a desk 13 and a floor 14 existing in an observation space, and its reflected wave is received by a microphone 102. Partial waveform data corresponding to a reference object is cut out by a common time window from reflected wave data derived by demodulating the received reflected wave, and phase characteristics are stored in memory for a prescribed time portion after frequency analyzing the cut-out data. A difference in phase characteristics between the stored partial waveform data is obtained in a round robin manner. When the temporal change is separated between a set greater than or equal to a prescribed threshold and a set less than the threshold, an object corresponding to the partial waveform data of the set less than the threshold is determined as being a quiescent object, that is, the object adoptable as a reference object.SELECTED DRAWING: Figure 2【課題】存在する物体の中から静止していると仮定できる物体を判別すること。【解決手段】観察空間内に存在する人体１２、机１３、床１４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。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所定時間数分だけ記憶させる。そして、記憶させた部分波形データ同士の位相特性の差を総当たりで求め、時間変化が所定の閾値以上の組と閾値未満の組に分かれたときに、時間変化が閾値未満の組の部分波形データに対応する物体が静止した物体、すなわち参照物体として採用可能な物体であると判定する。【選択図】図２
