A transistor with stable electrical characteristics is provided. Provided is a method for manufacturing a semiconductor device that includes, over a substrate, an oxide semiconductor, a first conductor, a first insulator, a second insulator, and a third insulator. The oxide semiconductor is over the first insulator. The second insulator is over the oxide semiconductor. The third insulator is over the second insulator. The first conductor is over the third insulator. The oxide semiconductor has a first region and a second region. To form the first region, ion implantation into the oxide semiconductor is performed using the first conductor as a mask, and then hydrogen is added to the oxide semiconductor using the first conductor as a mask.