We carried out a series of experiments on the culturing of micro-algae with the intention of eliminating carbon dioxide and recovering oxygen. The reactor used for culturing photosynthetic algae (photobioreactor) was manufactured on trial; we cultured C. ellipsoidea C-27 in this photobioreactor by passing air containing about 1% CO2 at a constant irradiation intensity of 1.6 W/reactor. The daily irradiation time was set for 8, 12, and 16 hours. The findings showed that the shorter the irradiation time, the longer it took to initiate growth (the growth lag phase increased), and the maximum number of cells obtained differed as well. On the other hand, the maximum growth rate remained almost unchanged, and the energy required for growth was accumulated in the cells regardless of the average daily dose of irradiation.As with the maximum growth rate, the maximum CO2 fixation rate was not influenced significantly by the irradiation conditions, although the chlorophyll per unit amount was most active at a daily irradiation dose of 8 hours. The amount of photoenergy given showed a correlation with the maximum amount of CO2 fixation In 12- and 16-hour daily irradiation. However, the amount of CO2 fixation per unit amount of photoenergy given in 8-hour daily irradiation was less than in 12- and 16-hour daily irradiation.When the daily dose was reduced to ½ in continuous irradiation, we did not observe any decreases in the growth rate or in the maximum number of cells obtained using air containing 5% CO2. This indicated that it is CO2 concentration that determines the amount of photoenergy required for cell growth.In addition, when the dose was reduced to 1/2 in continuous irradiation, the growth lag phase was much shorter, but the maximum photosynthesis was less active than in the 12-hour daily-irradiation experiment at an equivalent daily dose. The daily CO2 fixation amount was somewhat greater in the 12-hour irradiation experiment.