A Raman spectrum of a thin film which must be formed is measured in a thin-film forming step for forming the thin film on a member to be processed in an atmosphere, the pressure of which has been reduced. Moreover, the conditions under which the thin film is formed are controlled in accordance with a result of measurement of the Raman spectrum. At this time, the measurement of the Raman spectrum is continuously performed in an in-line manner while the thin film is being continuously formed on the elongated-sheet-like member to be processed. The measurement of the Raman spectrum is performed while the focal point of a probe of a Raman spectrometer is being controlled with respect to the member to be processed or while the output of a laser beam from the Raman spectrometer is being controlled. The thin film which must be. formed is, for example, a protective film of a magnetic recording medium. The protective film is, for example, a hard carbon film (a DLC film).