In a MIS transistor of which gate length is 10 nm or less, a gate insulator comprising a silicon oxide film formed on a silicon substrate and a high-k film formed on the silicon oxide film has a nitrided region including more nitrogen at the lateral side than at the central side in the gate-length direction, and including more nitrogen at the upper side than at the lower side in the film thickness direction. The reliability and characteristics of a MIS transistor using a gate insulator including a high-k (high dielectric constant) film is enhanced.