To provide a manufacturing method of a semiconductor device and an insulating film forming system, by which interfacial defects between a semiconductor substrate side and an insulating film are significantly reduced and the reliability as a device is made superior.

The manufacturing method of the semiconductor device, including a forming process of the insulating film, includes heat treatment in an atmosphere of a mixed gas of an inert gas and a nitric oxide gas, as one process of forming the insulating film, wherein nitric oxide gas of 0.5% or higher and 20% or lower, that is diluted by the inert gas, is contained.