A method of forming a silicon nitride film comprises: forming a silicon nitride film by applying first gas containing silicon and nitrogen and second gas containing nitrogen and hydrogen to catalyst heated in a reduced pressure atmosphere. A method of manufacturing a semiconductor device comprising the steps of: forming a silicon nitride film by the method as claimed in claim 1 on a substrate having the semiconductor layer, a gate insulation film selectively provided on a principal surface of the semiconductor layer, and a gate electrode provided on the gate insulation film; and removing the silicon nitride film on the semiconductor layer and the gate electrode and leaving a sidewall comprising the silicon nitride film on a side surface of the gate insulation film and the gate electrode by etching the silicon nitride film in a direction generally normal to the principal surface of the semiconductor layer. A method of manufacturing a semiconductor device comprising the steps of: forming a silicon nitride film by the method as claimed in claim 1 on a substrate including a semiconductor layer; forming an interlayer insulation layer on the silicon nitride film; forming a layer having an opening on the interlayer insulation layer; and etching the interlayer insulation layer via the opening in a condition where an etching rate for the silicon nitride film is greater than an etching rate for the interlayer insulation layer.