Specified frequency among the detection signals which are repeatedly detected at specified extraction period the cuffs pressure fluctuation velocity calculation expedient which calculates the fluctuation velocity of cuffs pressure the cuffs pressure detection expedient which detects the pressure inside cuffs 1 (2), cuffs pressure detection expedient (2) on the basis of the detection signal (14), by cuffs pressure detection expedient (2) difference of the cuffs pressure which is detected at the extraction period which leaves is calculated, the pulsating wave decision expedient which is decided whether or not cuffs pressure is the pulsating wave component that by comparing with the difference and reference level of the cuffs pressure which was calculated (16), and cuffs pressure fluctuation velocity calculation expedient (14) the reference level modification expedient which modifies reference level according to the fluctuation velocity of the cuffs pressure which was calculated by (15) it has. Detection of cuffs pressure is made possible even immediately after the modifying the reference level by applying the reference level before the modifying, pulsating wave decision expedient (16), only concerning 1 beats or predetermined time immediately after the modifying the reference level.カフ1内の圧力を検出するカフ圧検出手段(2)、カフ圧検出手段(2)の検出信号に基づいてカフ圧の変動速度を算出するカフ圧変動速度算出手段(14)、カフ圧検出手段(2)により所定の抽出周期で繰り返し検出される検出信号のうちの所定回数離れた抽出周期で検出されたカフ圧の差を算出し、その算出されたカフ圧の差と基準値とを比較することによってカフ圧が脈波成分であるか否かを判定する脈波判定手段(16)、およびカフ圧変動速度算出手段(14)により算出されたカフ圧の変動速度に応じて基準値を変更する基準値変更手段(15)を備える。脈波判定手段(16)は、基準値の変更直後の1拍または所定時間についてのみ、変更前の基準値を適用することにより、基準値の変更直後でもカフ圧の検出を可能とする。
