We have constructed a unit that uses microalgae to treat atmospheres containing low (i.e., (1000 ppm to 1%) concentrations of carbon dioxide. This experimental unit consists of a 20 l microalgae cultivation tank with eleven fluorescent lamps installed uniformly inside; and an analyzer with aeration/discharge, cultivation solutions supply/discharge function, and control capabilities. Using this unit, we conducted a series of experiments designed to reduce CO2 concentration in air from about 1000 ppm to atmospheric levels, and to regenerate oxygen. We used Chlorella sorokiniana ATCC22521, a species of green algae, which is said to have a relatively high growth rate. We studied a number of necessary conditions.We performed a series of experiments under various cultivation temperature conditions. Although temperature had some influence on specific growth rate, its influence was negligible on the equipment's maximum CO2 removal rate, as signified by represented an maximum cell growth or photosynthesis activity.We also performed a series of experiments under various pH conditions. We found that the CO2 removal rate rose at a pH value which exceeded the optimum growth pH. However, we did not see any significant influence of pH on algae growth.These findings suggest that the CO2 formation rate had a larger influence on the growth and photosynthetic activities of Chlorella sorokiniana ATCC22521 than other factors.Next, we conducted a series of experiments in which the oxygen concentration of the aeration gas was suppressed intermittently. When the oxygen concentration is suppressed, the equipment's CO2 removal rate went up quickly. We observed this phenomenon in every step of cell growth. Moreover, even in conditions when we did not suppress oxygen concentration, CO2-removal performance was improved relative to when the oxygen concentration of the aeration gas was not suppressed at all.We believe several reasons explain this phenomenon: 1) Since oxygen, a product of photosynthesis, is effectively removed, photosynthesis activities rose. 2) Because cellular photo-respiration was controlled effectively, apparent photosynthetic activity rose. 3) Decreasing the cell toxicity inherent to oxygen improved cellular activity. Hence, in order to improve the performance of an atmospheric air-adjusting unit which uses microalgae in a low-CO2 environment, it is important to control oxygen concentration. Moreover, control of oxygen concentration will likely enable the unit to be better controlled as well.