In forming a silicon nitride film by the low-pressure CVD method using a silane gas and ammonia, the reaction pressure is set to lie over a range of from about 0.05 to about 0.25 Torr, enabling a silicon nitride film to be formed, of uniform thickness, highly efficiently even on large wafers, and maintaining high yield, improved uniformity and good quality as a whole without decreasing the film-forming efficiency. Preferably, the reaction pressure is maintained over a range of from 0.1 to 0.2 Torr to further increase the efficiency, while preferably maintaining the temperature over a range of from 700.degree. to 1000.degree. C.