PURPOSE: To evolve hydrogen without using an expensive gas and without requiring a means to separate hydrogen gas, by using a gas produced by methane fermentation as an atmosphere gas for a device for evolving hydrogen photochemically using the blue-green algae.

CONSTITUTION: The blue-green algae cultivated and propagated in the culture tank 1 are separated and concentrated by the concentrating tank 2, and hydrogen is evolved by the hydrogen evolving tank 3. The blue-green algae after the evolution of hydrogen, having reduced ability to evolve hydrogen, is separated from the liquid with the algae sent from the separator 4, and subjected to methane fermentation by the methane fermentation tank 9. A digested gas evolved in the fermentation is removed from hydrogen sulfide, a corrosive gas contained in the digested gas by the desulfurizer 10, and the digested gas from which hydrogen sulfide is removed is fed to the hydrogen evolving tank 3 by the blower 7 in order to keep the hydrogen evolving tank 3 in an anaerobic state. While the digested gas containing hydrogen gas evolved by the hydrogen evolving tank 3 is stored in the gas tank 6.