In severe accidents of boiling water reactors (BWRs), the contaminated gas is to be released through a vent tube from a pressure vessel into a suppression pool for decompression of the vessel. The gas contains non-condensable gas, steam, and aerosolized radioactive materials. In the process of passing through pool water, radioactive particles contained in the gas are trapped from bubbles into pool water. This effect of capturing particles is called pool scrubbing, which is expected to have great decontamination effect. This effect can drastically decrease the amount of radioactive materials released into environment. Thus, it is important to understand the mechanism of pool scrubbing in analyzing the influence of severe accidents. But, currently, studies on the principal of scrubbing is not carried out sufficiently. For the purpose of elucidating the principle, in this study, we focused on bubble behavior such as bubbles formation, bubbles breakup, and changing of bubbles shape in the gasliquid two-phase flow. Air flow or air-steam mixture flow injected from L-shaped nozzle into the test water tank were measured by a wire mesh sensor (WMS) and visualized with a high speed camera. Measurement was conducted using gas flow rate, subcooling temperature, concentration ratio of condensable gas, and water depth as parameter under atmospheric pressure with water of ΔT_sub = 2 ~ 80 K. In nozzle exit region, when air was injected, large and unstable bubble called globule was formed and broken up into smaller bubbles with rising. On the other hand, when air-steam mixture gas was injected, the gas was broken up because of steam condensation behavior. As increasing steam flow rate, it was observed that condensation behavior was promoted. In swarm rise region where small bubbles are rising up, as comparing bubble size measured through the WMS, the difference between air and air-steam mixture gas was confirmed. However, since steam condensed completely, void fraction and gas rise velocity had no difference. Currently, decontamination factor (DF) of pool scrubbing is calculated by using analysis code such as MELCOR. In MELCOR, calculation of DF is conducted under the model using the gas-liquid two-phase model and bubble internal flow model. But the validity verification of the model is necessary for the accurate analysis of severe accident influence. Thus, we compared measurement results with physical model in MELCOR, and evaluated the individual decontamination elements of pool scrubbing.