A semiconductor photocatalyst includes first and second layers made of first and second materials, respectively. Band gaps of the first and second materials are equal to or smaller than 1.5 eV and 2.5 eV, respectively. A lower electric potential of a conduction band of the second material is disposed on a positive side from the first material. An upper electric potential of a valence band of the second material is disposed on a positive side from the first material and from an oxidation electric potential of water when the first and second layers are bonded to each other in the hetero junction manner. The lower electric potential of the conduction band of the first layer is disposed on a negative side from a reduction electric potential of hydrogen when the first and second layers are bonded to each other in the hetero junction manner.