A process of depositing a silicon nitride (SiN) film on a nitride semiconductor layer is disclosed. The process includes steps of: (a) loading an epitaxial substrate including the nitride semiconductor layer into a reaction furnace at a first temperature and converting an atmosphere in the furnace into nitrogen (N2); (b) raising the temperature in the furnace to a second temperature while keeping pressure in the furnace at a first pressure higher than 30 kPa; (c) converting the atmosphere in the furnace to ammonia (NH3) at the second temperature; and (d) beginning the deposition by supplying SiH2Cl2 as a source gas for silicon (Si) at a second pressure lower than 100 Pa. A feature of the process is that a time span from when the temperature in the furnace reaches the critical temperature to the supply of SiH2Cl2 is shorter than 20 minutes, where the first pressure becomes the equilibrium pressure at the critical temperature.