To provide a semiconductor device that enables to suppress a defect density of a gate insulating film of an MISFET, gain a sufficient electric characteristic thereof, and make an Equivalent Oxide Thickness (EOT) of the gate insulating film 1.0 nm or less. The MISFETs are formed to have the gate insulating film formed on a main surface of a silicon substrate, and a gate electrode formed on the gate insulating film, wherein the gate insulating film includes a metal silicate layer formed by a metal oxide layer and a silicon oxide layer and the metal silicate layer is formed so as to have concentration gradients of metal and silicon from a silicon substrate side toward a gate electrode side.