Provided are a photosynthesis promotion method and a phototroph cultivation method for promoting photosynthesis by radiating microwaves on an organism able to photosynthesize, for example. Photosynthesis is promoted by radiating microwaves on at least a portion where photosynthesis occurs. If, for example, the amount of light is insufficient due to weather or climate, microwaves are radiated on plants, which photosynthesize, thereby promoting photosynthesis in areas other than surfaces where light is received. Growth of plants can thereby be promoted. The present invention makes it possible to promote growth in plants and the like not only in extremely small spaces but also at extremely high energy efficiency, and to efficiently consume carbon dioxide and produce biofuels, foods, oxygen, medicines such as vitamins, and the like.
