The performances of a semiconductor device are improved. The device includes a first MISFET in which hafnium is added to the gate electrode side of a first gate insulation film including silicon oxynitride, and a second MISFET in which hafnium is added to the gate electrode side of a second gate insulation film including silicon oxynitride. The hafnium concentration in the second gate insulation film of the second MISFET is set smaller than the hafnium concentration in the first gate insulation film of the first MISFET; and the nitrogen concentration in the second gate insulation film of the second MISFET is set smaller than the nitrogen concentration in the first gate insulation film of the first MISFET. As a result, the threshold voltage of the second MISFET is adjusted to be smaller than the threshold voltage of the first MISFET.