A method of manufacturing a semiconductor device includes: providing a substrate that includes a surface exposing a first film containing silicon, oxygen, carbon and nitrogen and having an oxygen atom concentration higher than a silicon atom concentration, which is higher than a carbon atom concentration, which is equal to or higher than a nitrogen atom concentration; and changing a composition of a surface of the first film so that the nitrogen atom concentration becomes higher than the carbon atom concentration on the surface of the first film, by supplying a plasma-excited nitrogen-containing gas to the surface of the first film.