We have reported that Euglena, a photosynthetic alga, has the ability to grow in a high CO2 atmosphere and yield great biomass due to the induction of chlorophyll biosynthesis under high CO2 conditions. In the present study, we investigated some microalgal culturing systems for CO2 elimination and O2 regeneration in closed air on the ground by using the photosynthetic alga, Euglena gracilis. z.In the first part of our experiments, we focused on the effects of CO2 and light intensity in the present culture system. We investigated to determine the optimum culture conditions for a microalgal culture system that functions effectively in the controlled ecological life support systems (CELSS). We determined the maximum elimination rate under a high concentration of CO2 with stirring of the supply gas by bubbling. We found that the maximum CO2 elimination rate or gas exchange performance under a 10 % concentration of CO2 was about 2.3 times higher than its low concentration of CO2 (0.04 %). The results suggest that the CO2 concentration in the supply gas rate limits the algal system performance. In the second part of present experiments, we developed a bioregenerative system for converting CO2 to O2 which uses two functional membranes because centrifuge did not seem good for a micro algal culturing system under low gravity conditions. As ordinary atmospheric air contains about 0.04 % CO2, we introduced two different modules; one was a selective gas / water separation module which is able to separate CO2 specifically from the atmospheric air, and the other was a non-selective gas/water separation module which supplies O2 rich gas in the chamber after passing through the photobioreacti CO2 generator. At present, we conclude that introduction of selective membrane for the separation of O2 and CO2 is necessary and efficient under microgravity conditions.