To provide a manufacturing method of a semiconductor device capable of forming, as a protective film of an MTJ element, a silicon nitride film having good insulation properties without deteriorating the properties of the MTJ element. The method of the invention includes steps of forming a silicon nitride film over the entire surface including an MTJ element portion (MTJ element and an upper electrode) while using a parallel plate plasma CVD apparatus as a film forming apparatus and a film forming gas not containing NH3 but composed of SiH4/N2/helium (He). The film forming temperature is set at from 200 to 350° C. More ideally, a flow rate ratio of He to SiH4 is set at from 100 to 125.