A dopant diffusion method includes: diffusing a dopant element into a semiconductor through an oxide film. The dopant element is contained in a compound gas having a gas partial pressure of not less than 0.1 torr and not more than 800 torr. A temperature of the semiconductor is set less than 750° C. and not more than 950° C. A method of manufacturing a semiconductor device including a semiconductor with a dopant element diffused therein, the method includes: diffusing a dopant element into the semiconductor through an oxide film. The dopant element is contained in a compound gas having a gas partial pressure of not less than 0.1 torr and not more than 800 torr, and a temperature of the semiconductor is set less than 750° C. and not more than 950° C.