PURPOSE: To culture a large amount of marine blue-green algae, by adding a condensed phosphate with specific concentration to a culture solution, dissociating sufficient iron ion, maintaining a dissolution state and promoting growth of blue-green algae.

CONSTITUTION: 1×10-5W1×10-2mol./l sodium pyrophosphate, etc., is added to a culture solution, 1×10-5mol./l iron content is dissociated until the culture solution becomes pH 6W7, the culture solution is maintained in a dissolution state and marine blue-green algae are cultivated while paying attention to high temperature, high salt concentration, pH and aeration to give 1×103 algae/l blue-green algae. Whether blue-green algae grow in a healthy state or not is judged by dyestuff and greenish blue is taken care of. Preferably Guilard's F medium is fundamentally used as the medium.