PURPOSE: To carry out the photoproduction of hydrogen with hydrogen-producing photosynthetic microorganisms, for a long period, by adding a nitrogen source in the nighttime to the cultivation system of the microorganisms.

CONSTITUTION: In the photoproduction of hydrogen by the cultivation of photosynthetic bacteria having hydrogen-producing capability, e.g. Rhodospirillum rubrum ATCC11170, etc., a nitrogen source such as ammonium sulfate, nitrogen gas, yeast extract, etc. is added to the cultivation system in the nighttime, especially at the beginning of the nighttime.