A manufacturing method of an oxide semiconductor device includes the following steps. A first oxide semiconductor layer is formed on a substrate. A gate insulation layer is formed on the first oxide semiconductor layer. A first flattening process is performed on a top surface of the first oxide semiconductor layer before the step of forming the gate insulation layer. A roughness of the top surface of the first oxide semiconductor layer after the first flattening process is smaller than the roughness of the top surface of the first oxide semiconductor layer before the first flattening process.