Provided is an open diatom cultivation method where continuous production of one or more selected diatom species achieve dominancy in the production system and that dominancy is maintained. The algae culture solution containing diatoms is first introduced to an open microalgae culture system. Then, carbon dioxide and nutrient salts are added to form a culture solution for diatom cultivation. The method enables industrial scale cultivation of diatoms at economically competitive production cost, with a high cultivation density and for a high production volume. The method also enables continuous stable production of selected diatoms and solves the difficult problem of controlling invading algae by always using the most competitive of local diatoms without imposing a selected algae which may be superior in some localities and under some climate conditions.