A technique is provided which enables highly efficient hydrogen sulfide decomposition and hydrogen generation with a photocatalyst.It is a method of treating hydrogen sulfide or producing hydrogen which comprises disposing a liquid tank having a photocatalyst electrode 1 comprising a photocatalyst and a liquid tank having a metal electrode 2 so that the two liquid tanks are separated from each other by a cation-exchange membrane 3, placing a liquid containing either hydrogen sulfide or an organic substance in the liquid tank having the photocatalyst electrode 3, electrically connecting the photocatalyst electrode 3 to the metal electrode 2, and exposing the photocatalyst to a light. The liquid to be placed in the liquid tank having the metal electrode 2 preferably is an acidic solution. The photocatalyst preferably comprises a metal sulfide, and preferably is fine particles having a layered nanocapsule structure. The reactor may be one comprising an electrolytic cell 11 and a photoelectrochemical cell 34 disposed therein.