TECHNOLOGY FOCUS - BIOLOGY - Preferred Method: The method comprises a first step of obtaining the photosynthesis products under bright-aerobic conditions by the microalga, a second step, for obtaining the metabolic source for producing hydrogen from the photosynthesis product, under dark-anaerobic conditions using the microalga, and a third step for producing hydrogen from the metabolic source for producing hydrogen, by bacteria having the photosynthesis ability, where the microalga having a cell density elevated by the pre-culture is supplied to the first step and the bacteria having a cell density elevated by the pre-culture is supplied to the third step. Another method comprises a first step for obtaining the metabolic source for producing hydrogen, in the liquid phase under an osmosis pressure of 5.0 atm under bright-aerobic conditions by the microalga, and a second step, for producing hydrogen from the metabolic source for producing hydrogen, under bright-anaerobic conditions by the bacteria, where the microalga having a cell density elevated by the pre-culture is supplied to the first step and the bacteria having a cell density elevated by the pre-culture was supplied to the second step. During the step for obtaining the metabolic source for producing hydrogen, a gas containing 1.0 - 2.0 vol% of CO2 is flowed through the system. The microalga and the bacteria having the elevated cell density have their cell density adjusted to be 1.0 x 10 8 cells/ml or more, and are present in marine water or a supply source solution containing N2, inorganic components and vitamins in the saturated state. The supply source solution contains 1.0 - 10.0 wt% of salts. The bacteria are supplied in the state of a high concentration of nitrogenase that has an activity of 200 - 1000 nmol C2H4/mg cell dry wet/hour 30 - 50 mg(dry)/cm cubed (support) of the microalga is fixed on a support. The support can be a porous glass or porous resin, and is in the form of spheres having a diameter of 1 - 50 mm or cubes having a length of 1 - 50 mm, and having a pore-diameter of 10 - 500 microm. The microalga is a green alga such as Chlamydomonas reinhardtii, Chlamydomonas eugametos, Chlamydomonas segnis, Chlorella vulgaris, Senedesmus obliguus and Dunaliellatertrolecta or/and a blue alga such as Anabanena varuabilis, Cyanothece sp, Synecohoccus and Anacystis nidulans. The bacterium is Rhodopseudomonas palustris, Rhodospirillum rubrum, Rhodobacter sphaeroides or Rhodobacter capsulatus.Preferred Apparatus: The apparatus for producing hydrogen comprises a first bioreactor in which the photosynthesis products are obtained under bright-aerobic conditions by the microalga, a second bioreactor in which the metabolic source for producing hydrogen is obtained from the photosynthesis product under dark-anaerobic conditions by the microalga, and a third bioreactor in which hydrogen is produced from the metabolic source for producing hydrogen by bacteria, in which three bioreactors are connected by pipes or filters for flowing suspensions through them and the third bioreactor has a pipe for discharging hydrogen. EXAMPLE - An apparatus comprising two acryl resin bioreactors furnished with helical flow promoters and connected with pipes. Gas was introduced from the bottom through a plurality of holes, and marine water was used. Green alga: Chlamydomonas W-80 (1.8 x 10 to the power 8 cells/ml) supported on porous polyvinyl formal carriers produced lower molecular weight organic compounds in a first bioreactor (osmosis pressure = 24.6 atm) under blowing air containing CO2 and under irradiation with sunlight and light from a light source (18 W/m squared). The suspension containing the produced organic compounds was introduced to a second bioreactor. In the second bioreactor, Rhodvulum sulfidophilum W-1S bacteria (5.3 x 10 to the power 8 cells/ml) supported on porous polyvinyl formal carriers produced hydrogen from the organic compounds at a rate of 53 liter/(m squared)(hour) in a second bioreactor.