A coating agent which produces a film with photocatalytic properties, comprises composite photocatalyst particles of titanium dioxide and a compound inactive as a photocatalyst, and at least a binder. The photocatalyst particles achieve a ratio of decomposition of acetaldehyde (DWA) values of at least 80%. A BET specific surface area of the composite particles of titanium dioxide and a compound inactive as a photocatalyst is from 10 to 300 m.sup.2/g. The composite particles of titanium dioxide and a compound inactive as a photocatalyst are produced by a method comprising the steps of preparing a water based slurry of pH 3 to 5 comprising titanium dioxide, preparing a water based solution comprising a compound inactive as a photocatalyst, and reacting the slurry and the water based solution together at a pH of 4 to 10.