There is provided a semiconductor device, in which characteristics of the semiconductor device are improved by thinning a gate insulating film and a leak current can be reduced, and a manufacturing method thereof. An aluminum film which is a metal film is formed over a polycrystalline semiconductor film, and plasma oxidizing treatment is performed to the aluminum film, whereby an aluminum oxide film is formed by oxidizing the aluminum film, and a silicon oxide film is formed between the polycrystalline semiconductor film and the aluminum oxide film.