A respiratory phase determination apparatus that is robust against individual differences in breathing between measurement subjects and fluctuations in individual measurement subject's breathing and that can perform a respiratory phase determination without the need for calibration. The apparatus includes a first section feature amount extraction unit (106) that extracts a first section feature amount that represents the feature of the first breathing sound in the breathing section, and a second section feature that represents the feature of the second breathing sound in the breathing section. A second section feature quantity extraction unit (107) that extracts a quantity, a feature quantity relativization synthesis section (108) that calculates a synthesized feature quantity by relativizing the first section feature quantity and the second section feature quantity, and the vicinity A respiratory phase determination unit that compares the combined feature values of the two respiratory intervals located at, and determines whether the two respiratory intervals are the inspiratory phase or the expiratory phase based on the comparison result and a predetermined reference ( 109).被測定者間の呼吸の個人差および被測定者個人の呼吸の変動に対してロバストで、かつ、キャリブレーションの必要なく呼吸相判定を行える呼吸相判定装置。この装置は、呼吸区間における第１呼吸音の特徴を表す第１区間特徴量を抽出する第１区間特徴量抽出部（１０６）と、呼吸区間における第２呼吸音の特徴を表す第２区間特徴量を抽出する第２区間特徴量抽出部（１０７）と、第１区間特徴量と第２区間特徴量とを相対化して合成特徴量を算出する特徴量相対化合成部（１０８）と、近傍に位置する２つの呼吸区間の合成特徴量を比較し、比較の結果と、予め定めた基準とに基づいて、２つの呼吸区間が吸気相または呼気相であるかを判定する呼吸相判定部（１０９）と、を備える。
