To obtain a polyphenyl thioether excellent in heat resistance/oxidation resistance, weather resistance and ultraviolet light resistance especially to the sunlight.

A polyphenyl thioether formed by reacting a mercapto aromatic compound with a halogenated aromatic compound in the presence of a copper catalyst or a lubricating oil comprising the polyphenyl thioether is heat-treated in the presence of an alkali metal hydroxide and/or an alkaline earth metal hydroxide under reduced pressure to provide a purified polyphenyl thioether excellent in heat resistance/oxidation resistance and weather resistance.