The present invention provides a method of producing a composite photocatalyst having a remarkable water splitting activity, which is capable of efficiently loading a co-catalyst having a small particle size in a highly dispersed manner on a surface of an optical semiconductor. According to the present invention, the method of producing a composite photocatalyst from a plurality types of optical semiconductors includes a step of heating a solid-liquid mixture containing a solvent, a co-catalyst or a co-catalyst source, and a plurality of types of optical semiconductors by irradiating the solid-liquid mixture with microwave.