A method for restoring a porous surface of a dielectric layer formed on a substrate, includes: (i) providing in a reaction space a substrate on which a dielectric layer having a porous surface with terminal hydroxyl groups is formed as an outer layer; (ii) supplying gas of a Si—N compound containing a Si—N bond to the reaction space to chemisorb the Si—N compound onto the surface with the terminal hydroxyl groups; (iii) irradiating the Si—N compound-chemisorbed surface with a pulse of UV light in an oxidizing atmosphere to oxidize the surface and provide terminal hydroxyl groups to the surface; and (iv) repeating steps (ii) through (iii) to form a film on the porous surface of the dielectric layer for restoration.