PURPOSE: To enhance the hydrogen production efficiency by the photocatalytic decomposition of water by suspending fine powder of a composite semiconductor composed of a p-type semiconductor and an n-type semiconductor in water and irradiating light to generate oxygen and hydrogen.

CONSTITUTION: A container 8 is partially provided with a light transmitting window 9 made of glass or quartz. 100ml water 10 is held in the container 8, and fine powder of a semiconductor obtd. by dispersing GaP as a p-type semiconductor in TiO2 as an n-type semiconductor is suspended in the water with an agitator 12. The container 8 is tightly sealed with a cover 14 having an outlet 13 leading out a generated gas. Light 4 with energy above the forbidden band of the fine semiconductor powder 11 is irradiated through the window 9. After the irradiation a gaseous mixture of oxygen with hydrogen is taken out of the outlet 13. As said semiconductor an n-type semiconductor with a p-type one dispersed on part of the surface or a p-type semiconductor with an n-type one dispersed on part of the surface is used.