Provided is a method for controlling a donor concentration in a Ga2O3-based single crystal body. In addition, an ohmic contact having a low resistance is formed between a Ga2O3-based single crystal body and an electrode. A donor concentration in a Ga2O3-based single crystal body is controlled by a method which includes a step wherein Si, which serves as a donor impurity, is introduced into the Ga2O3-based single crystal body by an ion implantation method at an implantation concentration of 1×1020 cm−3 or less, so that a donor impurity implanted region is formed in the Ga2O3-based single crystal body, the donor impurity implanted region having a higher donor impurity concentration than the regions into which Si is not implanted, and a step wherein Si in the donor impurity implanted region is activated by annealing, so that a high donor concentration region is formed.