A method for manufacturing a semiconductor device, comprising the steps of: i.) forming an oxide semiconductor film over a gate electrode with a gate insulating film provided therebetween; and either comprising the following steps ii.) to vi.) ii.) etching the oxide semiconductor film to form an island-shaped oxide semiconductor film so as to overlap the gate electrode with the gate insulating film provided therebetween, iii.) performing a first heat treatment on the island-shaped oxide semiconductor film, iv.) adding oxygen with an ion beam into the island-shaped oxide semiconductor film subjected to the first heat treatment, v.) performing a second heat treatment on the island-shaped oxide semiconductor film added with the oxygen, and vi.) forming source and drain electrodes over the island-shaped oxide semiconductor film (108, 153) subjected to the second heat treatment; or else comprising the following steps ii.) to vi.) ii.) performing a first heat treatment on the oxide semiconductor film, iii.) adding oxygen with an ion beam into the oxide semiconductor film subjected to the first heat treatment, iv.) performing a second heat treatment on the oxide semiconductor film added with the oxygen; v.) etching the oxide semiconductor film subjected to the second heat treatment to form an island-shaped oxide semiconductor film so as to overlap the gate electrode with the gate insulating film provided therebetween, and vi.) forming source and drain electrodes over the island-shaped oxide semiconductor film (108, 153) subjected to the second heat treatment wherein the first heat treatment and the second heat treatment are respectively performed at a temperature higher than or equal to 300 °C and lower than or equal to 850 °C.. i.) forming an oxide semiconductor film over a gate electrode with a gate insulating film provided therebetween; and either comprising the following steps ii.) to vi.) ii.) etching the oxide semiconductor film to form an island-shaped oxide semiconductor film so as to overlap the gate electrode with the gate insulating film provided therebetween, iii.) performing a first heat treatment on the island-shaped oxide semiconductor film, iv.) adding oxygen with an ion beam into the island-shaped oxide semiconductor film subjected to the first heat treatment, v.) performing a second heat treatment on the island-shaped oxide semiconductor film added with the oxygen, and vi.) forming source and drain electrodes over the island-shaped oxide semiconductor film (108, 153) subjected to the second heat treatment; ii.) performing a first heat treatment on the oxide semiconductor film, iii.) adding oxygen with an ion beam into the oxide semiconductor film subjected to the first heat treatment, iv.) performing a second heat treatment on the oxide semiconductor film added with the oxygen; v.) etching the oxide semiconductor film subjected to the second heat treatment to form an island-shaped oxide semiconductor film so as to overlap the gate electrode with the gate insulating film provided therebetween, and vi.) forming source and drain electrodes over the island-shaped oxide semiconductor film (108, 153) subjected to the second heat treatment