A manufacturing method of a semiconductor device includes: preparing a semiconductor substrate including a first semiconductor layer made of gallium oxide containing Sn and a second semiconductor layer disposed on the first semiconductor layer and made of n type gallium oxide having a Sn concentration lower than a Sn concentration of the first semiconductor layer; implanting ions of a group 2 element into the second semiconductor layer; and forming a diffusion region, in which the group 2 element diffuses, in a range from a surface of the second semiconductor layer to an interface between the second semiconductor layer and the first semiconductor layer.