In ethyl alcohol production using the self-fermentation of a microalga, a step of concentrating or collecting an algal body by centrifugal treatment, filtering treatment or the like is made unnecessary or simple to save labor for effort and equipment therefor is saved. The microalga belongs to Chlamydomonas sp., and is a variant strain which has an ability to produce ethyl alcohol under dark and anaerobic conditions and has acquired an ability to proliferate while aggregating. The microalga is proliferated and maintained under dark and anaerobic conditions to generate ethyl alcohol in this method for producing ethyl alcohol.