While green algae are expected to serve as raw materials of biomass fuels, they are damaged by high-intensity light when subjected to mass-culture outdoors in summer, and biomass productivity is deteriorated as a consequence. In order to overcome such a drawback, the present invention provides a high-intensity light resistant green algae mutant that can be subjected to outdoor culture in summer. Specifically, the present invention relates to such green algae mutant, wherein functions or expression levels of a protein having a response regulatory domain at the N-terminus and a WD40 domain at the C-terminus are lower than those in a wild-type strain, and wherein said green algae mutant grows faster than a wild-type strain when cultured at a light intensity of 1,000, 1,500, or 2,000 μmol photons m−2 s−1 measured as photosynthetically active radiation (PAR).