PURPOSE: To provide the reactor capable of suppressing temp. rising therein can be controlled and being stirred sufficiently.

CONSTITUTION: This reactor 16 which contains photosynthetic microorganisms capable of producing hydrogen by decomposing organic matter in waste water through utilization of sunlight, is installed in a floating state on a water surface 11 such as sea surface, in which a way that a roof section 21 of the reactor 16 is formed so as to have a multiple chevron shape and the lower part of the roof section 21 is half sunk under the water surface 11.