An object is to provide a semiconductor device including an oxide semiconductor, which has stable electrical characteristics and improved reliability. In a transistor including an oxide semiconductor film, insulating films each including a material containing a Group 13 element and oxygen are formed in contact with the oxide semiconductor film, whereby the interfaces with the oxide semiconductor film can be kept in a favorable state. Further, the insulating films each include a region where the proportion of oxygen is higher than that in the stoichiometric composition, so that oxygen is supplied to the oxide semiconductor film; thus, oxygen defects in the oxide semiconductor film can be reduced. Furthermore, the insulating films in contact with the oxide semiconductor film each have a stacked structure so that films each containing aluminum are provided over and under the oxide semiconductor film, whereby entry of water into the oxide semiconductor film can be prevented.