A transistor including an oxide semiconductor with favorable electric characteristics and a manufacturing method thereof are provided. A semiconductor device includes a transistor. The transistor includes an oxide semiconductor film over a base insulating film, a gate electrode overlapping with the oxide semiconductor film with a gate insulating film interposed therebetween, and a pair of electrodes in contact with the oxide semiconductor film and serving as a source electrode and a drain electrode. The base insulating film includes a first oxide insulating film partly in contact with the oxide semiconductor film and a second oxide insulating film in the periphery of the first oxide insulating film. An end portion of the oxide semiconductor film which crosses the channel width direction of the transistor is located over the second oxide insulating film.