Both high cost and environmental load of surfactant are issues to be solved in foam EOR. Moreover, it is difficult to control the injection of surfactant and gas so that the foam is generated in only high permeable zones selectively in oil reservoir. The authors have found a foam generating microorganism and hit upon an idea of the microbial foam EOR which makes the microorganism do generating foam in oil reservoir. The mechanism of the microbial foam generation and culture condition suitable for the foam generation were studied in this study. A species of Pseudomonas aeruginosa was used as a foam producer in this study. It was cultured in the medium consisting of glucose and eight kinds of minerals at 30 °C and atmospheric pressure under anaerobic conditions. Because P. aeruginosa generally grows better under aerobic conditions, the microorganism was supplied with oxygen nanobubbles as the oxygen source. The carbon dioxide nanobubbles were also used as a comparison target in this study. The state of foam generation in the culture solution was observed during the cultivation. The surface tension, surfactant concentration, protein concentration, polysaccharides concentration and bacterial population of the culture solution were measured respectively. The foam was started to be generated by the microorganism after 2 days of cultivation and its volume became maximum after 3 days of cultivation. The foam generation was found in the culture solution which contained both oxygen nanobubbles and carbon dioxide nanobubbles whereas little foam was found in non-nanobubbles culture solution. The foam generation found in the culture solution containing carbon dioxide nanobubbles was more than that in the culture solution containing oxygen nanobubbles. Both gas and protein concentration increased along with the formation of the foam whereas surfactant and polysaccharides were not increased, therefore, the foam was assumed to be generated with gas and protein which were generated by P. aeruginosa. It was found that the carbon dioxide nanobubbles were positively charged in the culture medium whereas they were negatively charged in tap water through the measurement of zeta potential of nanobubbles, therefore, the carbon dioxide nanobubbles attracted cations in the culture medium and became positively charged. Positively charged carbon dioxide nanobubbles transported cations to the microbial cells of P. aeruginosa. Among cations in the culture medium, ferrous ions are essential for the protein generation of P. aeruginosa, therefore, the positively charged carbon dioxide nanobubbles attracted ferrous ions and transport them to the microbial cells, resulting the growth and metabolism of P. aeruginosa were activated. Those results suggest that the microbial foam EOR can be materialized by supplying the microorganism with carbon dioxide nanobubbles or ferrous ions.