The mixing process in an agitated vessel with aeration was investigated experimentally by injecting tracer gas impulsively into the vessel. On the basis of the change in concentration of absorbed tracer gas in water with time, the change in the mixedness based on the information entropy with time was calculated. The mixing process is made up of three steps; the first step is associated with rise of gas from the nozzle to the impeller, the second step is associated with one circulation of discharge flow from the impeller, and the third step is associated with the circulation flow and fluctuating flow.