A manufacturing method of a semiconductor device includes forming an oxide semiconductor layer on an insulating layer, a part of the insulating layer being exposed from the oxide semiconductor layer, performing a plasma process by use of chlorine-containing gas on the part of the insulating layer exposed from the oxide semiconductor layer, and removing chlorine impurities from a surface layer of the exposed part of the insulating layer. The chlorine impurities may be removed by a first etching process performed by use of fluorine-containing gas. The fluorine-containing gas may contain CF4 and CHF3. The plasma process may be a second etching process performed by use of chlorine-containing gas.