To provide a photocatalyst having an excellent light transmittance and a lamp and an illumination device using the catalyst, which can be used as a photocatalyst film by a ultrafine particle coating method and has an improved adhesion property onto a substrate of the photocatalyst film and a high film strength so that the film is hardly damaged.

A metal oxide structural layer 3 having a large number of trough holes 3a is formed on a photocatalyst film 2 carried on a substrate. The photocatalyst film 2 can be formed by coating a dispersion of titanium oxide ultrafine particles and burning. The metal oxide structural layer 3 that is prepared by using at least one of silicon, aluminum and titanium is effective. The plurality of the through holes 3a are formed through decomposition and evaporation of synthetic resin particles at the time of burning by adding e.g. 30 wt.% or more of the synthetic resin particles to a coating liquid so as to form through holes 3a in the sites of the particles. A substance to be decomposed passes through the holes 3a in the metal structural layer 3 and is brought into contact with a photocatalyst film and decomposed.