To easily and effectively ensure amt. of light receiving of a photocatalyst and reaction area with a simple structure by providing a light source and fixing plates for a photocatalyst in a reaction tank and arranging the fixing plates for the photocatalyst at positions being approximately orthogonal planes to the light source.

A lamp 3 is lighted to irradiate fixing plates 2 for a photocatalyst with a specified amt. of light and to excite the photocatalyst on the surfaces. A fluid to be treated is fed into a reaction tank 1 from an introducing hole 1a and decomposition and removing treatments are performed by the excited photocatalyst. In the vicinity of the lamp 3 for the fixing plates 2 for the photocatalyst, i.e., in the vicinity of a through-hole 2a of the fixing plates 2 for the photocatalyst, the fluid is treated by utilizing effectively the attenuated weak light even at a place being far from the lamp 3. Even when light transmittance of the fluid to be treated is low, in the vicinity of the through-hole 2a of the fixing plates 2 for the photocatalyst, strong light of the lamp 3 is surely received and there exists no decrease in treating efficiency. The treated fluid is discharged to the outside of the reaction tank 1 from an outlet 1b. As a result, it is possible to increase treating efficiency of the fluid and to simplify maintenance and control of the apparatus by its simple structure.