Observations of bubbles in the water seal and clamp tests have been employed for the judgment on the removal timing of the chest tubes. These methods tend to depend on the observers, therefore methods for quantitative assessment of air leak have been desired. We have investigated the correlation between the occurrence of bubbles in the water seal and the pressure fluctuation in the flow path of the electrically powered suction equipment and found that the bubbles can be detected according to the following criteria on the pressure fluctuation: ①continuous pressure increase or decrease over 0.02s, ②2kPa/s or faster pressure increase or decrease during above period, ③continuous satisfaction of conditions ① and ② except interruption of 0.2s or shorter. Judged bubbling time detected with above criteria was well correlated with bubbling time observed in the water seal. The judged bubbling time can be correlated with the air leak flow rate by taking account for the effect of the set suction pressure of the suction equipment.