PURPOSE: To form a silicon nitride film having a uniform thickness distribution and uniform quality distribution by specifying the flow of silane gas to be supplied between a substrate and the boundary of a plasma generation region and a reaction region.

CONSTITUTION: A gas release port 311 of a double-cylinder type cylindrical body 301 near a plasma generating chamber 10 is so located as to release gas right below an exit of the plasma generating chamber 10 and a gas release port 312 of another double-cylinder type cylindrical body 302 near a substrate 51 is so located as to release gas right before the substrate. Silane gas is supplied to an inner space of each cylindrical body through gas flow controllable mass-flowmeters 313, 314 from a common silane gas source and then the silane gas is supplied between the exit of the plasma generating chamber 10 and the substrate 51 through the gas release ports 311, 312. At the time of film formation, the gas flow is so controlled that the gas flow passing through the mass-flowmeter 313 may be three times as much as the gas flow passing through the mass-flowmeter 314. By this method, a silicon nitride film having a uniform thickness distribution and good quality can be obtained.