To provide a new manufacturing method of a semiconductor device which materializes the suppression of microloading effect and etching at high selectivity ratio while curtailing the quantity of discharged PFC gas, and besides is high in productivity, in the fine processing by dry etching of a silicon nitride film (Si3N4).

This is the manufacturing method of a semiconductor device which forms a gate electrode through a gate oxide film 2 on a semiconductor substrate 1, grows a silicon oxide film 6 on the semiconductor device consisting of source and drain diffusion layers 1a and 2b provided to catch this gate electrode, further grows a silicon nitride film 7 on this silicon oxide film 6, and forms a sidewall at the gate electrode. This includes at least a first process of etching the silicon nitride film 7 using the etching gas 8 including a halogen gas or a hydrohalide gas until the silicon oxide film 6 is exposed, and a second process of etching the gate oxide film 2 and the silicon oxide film 6 using the mixed gas 9 between fluorocarbon gas and He until the diffusion layers 1a and 1a are exposed.