A method for manufacturing a semiconductor device is disclosed which enables to suppress decrease in the mobility in a channel region by suppressing piercing of boron through a gate insulation film which boron is ion-implanted into a gate electrode. The method for manufacturing a semiconductor device includes: a step for forming a gate insulating layer on an active region of a semiconductor substrate; a step for introducing nitrogen through the front surface of the gate insulating layer using active nitrogen; and a step for conducting an annealing treatment in an NO gas atmosphere so that the nitrogen concentration distribution in the nitrogen-introduced gate insulating layer is high on the front surface side and low on the side of the interface with the semiconductor substrate.