A method of forming a nitrogen-incorporated silicon or metal oxide film, includes (i) depositing by a plasma a silicon or metal oxide film on a substrate using a precursor containing a silicon or metal and an oxidizing gas, said plasma having a first plasma density; and (ii) nitriding by a plasma the silicon or metal oxide film using a nitriding gas without using any precursor, said plasma having a second plasma density which is higher than the first plasma density.