To provide a photocatalyst decomposition apparatus that can supply a liquid phase containing a substance to be decomposed by a photocatalyst and that can perform decomposition of the substance more efficiently than in the related art. A photocatalyst decomposition system according to the invention includes: a gas phase generation apparatus configured to convert a liquid phase containing a decomposition object into a gas phase; and a photocatalyst member configured to come into contact with the gas phase to decompose the decomposition object by light from a light source. The photocatalyst member includes a base material formed of a porous material and a photocatalyst layer provided on a surface of the base material.