Disclosed is a respiratory phase determination apparatus enabling a respiratory phase determination robust to personal differences in respiration among subjects and fluctuations in respiration of subjects in person without any need for calibration. The apparatus includes a first period feature value extraction section that extracts a first period feature value representing a feature of a first breath sound in a respiratory period, a second period feature value extraction section that extracts a second period feature value representing a feature of a second breath sound in the respiratory period, a feature value relativization/combining section that relativizes the first and the second period feature values and calculates a combined feature value, and a respiratory phase determination section that compares combined feature values of two neighboring respiratory periods and determines whether the two respiratory periods are each an inspiratory phase or expiratory phase based on the comparison result and a predetermined datum.
