To provide a semiconductor element in which generation of oxygen vacancies in an oxide semiconductor thin film can be suppressed. The semiconductor element has a structure in which, in a gate insulating film, the nitrogen content of regions which do not overlap with a gate electrode is higher than the nitrogen content of a region which overlaps with the gate electrode. A nitride film has an excellent property of preventing impurity diffusion; thus, with the structure, release of oxygen in the oxide semiconductor film, in particular, in the channel formation region, to the outside of the semiconductor element can be effectively suppressed.