To provide good interface properties without allowing contamination of an impurity such as moisture in a thin film transistor having an oxide semiconductor layer; and provide a semiconductor device having a thin film transistor of high electrical characteristics and reliability; and provide a method of manufacturing the semiconductor device with high mass productivity.SOLUTION: To perform an oxygen radical treatment on a surface of a gate insulation layer is a subject matter. Accordingly, a semiconductor device has an oxygen concentration peak at an interface between the gate insulation layer and a semiconductor layer; and an oxygen concentration of the gate insulation layer has a concentration gradient and the oxygen concentration increases with the decreasing distance from the interface between the gate insulation layer and the semiconductor layer.SELECTED DRAWING: Figure 1