Provided is a semiconductor device having favorable reliability. A manufacturing method of a semiconductor device comprising the steps of: forming a first oxide semiconductor having an island shape; forming a first conductor and a second conductor over the first oxide semiconductor; forming an oxide semiconductor film over the first oxide semiconductor, the first conductor, and the second conductor; forming a first insulating film over the oxide semiconductor film; forming a conductive film over the first insulating film; removing part of the first insulating film and part of the conductive film to form a first insulator and a third conductor; forming a second insulating film covering the first insulator and the third conductor; removing part of the oxide semiconductor film and part of the second insulating film to form a second oxide semiconductor and a second insulator and to expose a side surface of the first oxide semiconductor; forming a third insulator in contact with the side surface of the first oxide semiconductor and with a side surface of the second oxide semiconductor; forming a fourth insulator in contact with the third insulator; and performing a microwave-excited plasma treatment to the third insulator and the fourth insulator.