To provide a method and apparatus for cleaning a substrate to effectively remove an organic type or metallic type contaminant from a to-be-cleaned surface of a substrate by an increase in the intensity of UV light at the to-be-cleaned surface of the substrate and by an increase in the concentration of ozone O.sub.3, excited state oxygen atoms O(1D) and active oxygen.A method for cleaning a substrate by means of irradiation with UV light, wherein a second space containing a surface on the side of the substrate on which the cleaning is to be carried out (hereinafter referred to as the to-be-cleaned surface) and its vicinity, is an atmosphere comprising a gas or a liquid which generates at least one of ozone, excited state oxygen atoms and active oxygen species by the irradiation with the UV light, a first space containing a surface on the other side of the substrate (hereinafter referred to as the to-be-irradiated surface) is an atmosphere comprising a gas showing a low absorption of the UV light, and the UV light is permitted to enter via the first space into the to-be-irradiated surface of the substrate, then pass through the interior of the substrate and then be applied to the to-be-cleaned surface.