PROBLEM TO BE SOLVED: To provide a technology improving accuracy in calculation of intrathoracic pressure.SOLUTION: An intrathoracic pressure calculation apparatus obtains pulse wave signal (S230), and then obtains mouth pressure signal representing level of mouth pressure of a subject who carries out respirations having different depth along with time base as mouth pressure signal associated along with pulse wave signal and time (S210). Based on the obtained mouth pressure signal and pulse wave signal, the apparatus calculates, as calibration coefficient, ratio of change in change amount from a preset second criterion value of amplitude in the pulse wave signal relatively to change in change amount from a preset first criterion value of the mouth pressure represented by the mouth pressure signal (S260, S270). In addition, the apparatus calculates absolute value of subject's intrathoracic pressure by multiplying expected intrathoracic pressure as relative value of intrathoracic pressure expected based on the pulse wave signal by calibration coefficient.SELECTED DRAWING: Figure 6【課題】胸腔内圧の算出精度を向上させる技術の提供。【解決手段】胸腔内圧算出装置は、脈波信号を取得し（Ｓ２３０）、被験者が時間軸に沿って深さの異なる呼吸を行った場合の当該被験者の口腔内圧の大きさを表す口腔内圧信号であって脈波信号と時間軸に沿って対応付けられた口腔内圧信号を取得する（Ｓ２１０）。その取得した口腔内圧信号と脈波信号とに基づいて、口腔内圧信号によって表される口腔内圧の予め設定された第１基準値からの変化量の変動量に対する、脈波信号の振幅の予め設定された第２基準値からの変化量の変動量の比率を、キャリブレーション係数として算出する（Ｓ２６０，Ｓ２７０）。そして、脈波信号に基づいて推定した胸腔内圧の相対値である推定胸腔内圧にキャリブレーション係数を乗ずることで、被験者の胸腔内圧の絶対値を算出する。【選択図】図６
