A signal waveform of an estimation signal and a signal waveform of the reflected light intensity signal are coordinated with each other such that a time point when a film thickness is equal to zero in the signal waveform of the estimation signal and a base point in the signal waveform of the reflected light intensity signal coincide with each other. A film thickness corresponding to that estimated value of a signal intensity of a reflected light which corresponds to a film thickness range corresponding to a time range in the signal waveform of the estimation signal and coincides with the signal intensity of the reflected light at a desired time point is set as a film thickness of a thin film at the desired time point.