A respiratory phase determination apparatus comprising: a first sensor, attached at a first position of a body of a subject, configured to acquire a first biological sound including a vesicular breath sound; a second sensor, attached at a second position of the body of the subject, configured to acquire a second biological sound including a bronchial breath sound; a first breath sound extraction section configured to extract the vesicular breath sound from the first biological sound; a second breath sound extraction section configured to extract the bronchial breath sound from the second biological sound, the bronchial breath sound acquired in synchronization with the vesicular breath sound; a respiratory period estimation section configured to estimate a respiratory period based on at least one of the vesicular breath sound and the bronchial breath sound; a first period feature value extraction section configured to calculate a first period feature value indicating average power of the vesicular breath sound in the respiratory period; a second period feature value extraction section configured to calculate a second period feature value indicating average power of the bronchial breath sound in the respiratory period; a feature value relativization/combining section configured to calculate a combined feature value by subtracting the first period feature value from the second period feature value; and a respiratory phase determination section configured to compare the combined feature values for a pair of respiratory periods, and to determine the respiratory period of a greater combined feature value to be an expiratory phase and to determine the respiratory period of a smaller combined feature value to be an inspiratory phase. a first sensor, attached at a first position of a body of a subject, configured to acquire a first biological sound including a vesicular breath sound; a second sensor, attached at a second position of the body of the subject, configured to acquire a second biological sound including a bronchial breath sound; a first breath sound extraction section configured to extract the vesicular breath sound from the first biological sound; a second breath sound extraction section configured to extract the bronchial breath sound from the second biological sound, the bronchial breath sound acquired in synchronization with the vesicular breath sound; a respiratory period estimation section configured to estimate a respiratory period based on at least one of the vesicular breath sound and the bronchial breath sound; a first period feature value extraction section configured to calculate a first period feature value indicating average power of the vesicular breath sound in the respiratory period; a second period feature value extraction section configured to calculate a second period feature value indicating average power of the bronchial breath sound in the respiratory period; a feature value relativization/combining section configured to calculate a combined feature value by subtracting the first period feature value from the second period feature value; and a respiratory phase determination section configured to compare the combined feature values for a pair of respiratory periods, and to determine the respiratory period of a greater combined feature value to be an expiratory phase and to determine the respiratory period of a smaller combined feature value to be an inspiratory phase.