To efficiently form a GaN compound semiconductor thin film, which is superior in film quality by a method, wherein nitrogen gas is brought into contact with a substrate in a plasma state and the plasma state nitrogen gas, which is successively fed, and the purposed gas produced from a organometallic compound containing each of the constituent elements of a compound semiconductor are brought into contact with the substrate.

The interior of a film-forming chamber 1 is evacuated to the state of a sufficiently high degree of vacuum and thereafter, the temperature of a substrate 7 is made to rise to 850°C, for example, by a resistance heating heater 5. Subsequently, only nitrogen gas, which is used as a raw gas of a second system, is fed into the chamber 1. Moreover, the nitrogen gas is fed in such a way as to come into contact with the surface of the substrate 7 in a plasma state. By this treatment, the surface of the substrate 7 is nitrided. Then, after the temperature of the substrate 7 has been raised to 900°C, for example, trimethyl gallium gas produced using hydrogen gas as carrier gas is brought into contact with the substrate 7 as the raw gas 8 of a first system, and the nitrogen gas is brought into contact with the substrate 7 in a plasma state as a raw gas 9 of the second system. Thereby, a GaN thin film is deposited on the substrate 7.