Disclosed is a semiconductor device manufacturing method in which a silicon nitride film is formed to cover an n-channel transistor formed on a semiconductor substrate and to apply a tensile stress in a channel length direction to a channel of the n-channel transistor, the method includes: forming a first-layer silicon nitride film above the n-channel transistor; irradiating the first-layer silicon nitride film with ultraviolet radiation; and after the ultraviolet irradiation, forming at least one silicon nitride film thinner than the first-layer silicon nitride film above the first-layer silicon nitride film. Silicon nitride films formed to apply the tensile stress is formed by respective steps.