A bioreactor equipped with a sunlight-collection device has been developed, for the production of microalgal biomass using sunlight. The bioreactor comprised a mirror-type sunlight collection device to collect sunlight, quartz fibers for transmission of the light, illumination plates made of acrylic resin for irradiation to cells, and an airlift fermenter. This bioreactor facilitated the culture of microalgal cells in a closed system. The efficiency of light collection, including transmission and irradiation in the bioreactor, was achieved up to 30% on the basis of photosynthetically active radiation. Under photoautotrophic conditions, continuous culture of Chlorella cells was successfully carried out for 14 days with regulation of flow rate of fresh medium in response to the change in solar irradiance. The cell concentration in the bioreactor could be kept almost constant for the culture period. The average production rates of the cells per unit area of sunlight collection was 0.20 X 10(-3) kg m(-2)h(-1), and the efficiency of conversion from light energy to chemical energy of cells was 9.8% based on a photosynthetically active radiation.