Provided is a manufacturing method of a semiconductor device including a vertical MOSFET having a planar gate. The manufacturing method of a semiconductor device includes forming a n-type gallium nitride layer on a gallium nitride monocrystalline substrate, and forming an impurity-implanted region that contains impurities at a uniform concentration in a direction parallel to a main surface of the gallium nitride monocrystalline substrate, by ion-implanting the impurities into the n-type gallium nitride layer, where the impurities include at least one type selected from among magnesium, beryllium, calcium and zinc. Here, at least part of the impurity-implanted region serves as a channel forming region of the vertical MOSFET.