PURPOSE: To decompose water by a simple structure and to efficiently generate hydrogen by applying a catalytic action by electrolysis and solar energy or the like to a substance to be treated to generate hydrogen such as liq. sludge.

CONSTITUTION: Liq. sludge 21 or a substance to be treated to generate hydrogen such as hydrocarbon or iodide is charged into a vessel 1. A pipe 22 provided with a heat pipe 23 is put in the vessel 1 to deprive the sludge of its heat, and the heat is stored in a regenerator 20. The sludge 29 is filled into a heating chamber 11 through a screw 26. A titanium electrode 33A and a cathode 33B are attached to the chamber 11, and an electrode 4 is turned on. At this time, heat from the regenerator 20 is utilized to generate electric power, and the electric power is supplied to the electrode 4. The sludge 29 in the chamber 11 is isolated from the open air by the light transmitting surface 31, absorbs solar energy 32, and evaporates water. Electrolysis using the supplied current causes a catalytic action, reaction under light energy is accelerated, and hydrogen is generated. The gaseous reaction product is captured in an upper gas chamber 5, fed to a vessel 7, freed of CO2 through an alkali soln. 39, and stored in a vessel 9 through a compressor 8.