An air leak detection device (5) is applied to a thoracic cavity drainage system (1A) which, using negative pressure generated by a vacuum source (4), aspirates gas in the thoracic cavity of a patient via a water seal chamber (11) in which liquid is received. The flow rate between the vacuum source (4) and the water seal chamber (11) is measured by a flow rate sensor (15), and the occurrence of air bubbles created in the water seal chamber (11) is detected on the basis of fluctuations of this flow rate.
