A method and an apparatus form a diamond film from a microwave plasma by controlling a manufacturing condition based on a spectroscopic measurement of the plasma light emission to obtain a large area of a high-quality diamond film. In the method of forming a diamond film, a gas mixture of hydrocarbon gas and hydrogen gas is introduced into a reactor, where the gas mixture is excited by microwave energy which is also introduced into the reactor to generate a plasma, and the light emitted from the plasma is spectroscopically measured. Furthermore, a formation condition of the diamond film is controlled such that the spectrum of a carbon molecule (C2) falls within a predetermined range of requirement. A carbon molecule vibration temperature is determined from the spectrum, and the formation pressure, or the gas flow rate is controlled so that the determined vibration temperature falls within a specified range, especially 2000 to 2800 K.