A manufacturing method of a semiconductor device, comprises; a process of heat-treating a semiconductor substrate under the ordinary pressure and in an oxidizing atmosphere; and a process of heat-treating the semiconductor substrate under the ordinary pressure and in an inert atmosphere, wherein heat-treating time or heat-treating temperature in heat treatment in the oxidizing atmosphere is changed based on the fluctuation of atmospheric pressure, and the heat-treating time in the inert atmosphere is determined based on the heat-treating time or the heat-treating temperature in the oxidizing atmosphere.