PURPOSE: To obtain gaseous oxygen and gaseous hydrogen in a separate state by attaching a window for irradiating light to one side of a semiconductor plate to a water container divided by the plate into 2 chambers each provided with a gas leading outlet and communicating with each other at the bottom part.

CONSTITUTION: A container 11 is a one for holding water at the inside, and it is divided into an oxygen generating chamber 13 and a hydrogen generating chamber 14 at the central part by a vertical wall 12 suspended from the ceiling of the chamber. Said 2 chambers communicate with each other at the bottom part so that liq. is circulated through the container. A light transmitting window 18 for irradiating light to a semiconductor plate 15 from the outside is attached to the side wall 17 of the container 11 at the side of the chamber 13. The upper parts of the chambers 13, 14 are provided with gas leading outlets 19, 20 for leading out generated gaseous oxygen and gaseous hydrogen, respectivity.