A film thickness measuring method comprising: arranging a volatile light-permeable film to be measured on a substrate, and irradiating the light-permeable film with light from a light source until an end of evaporation of the light-permeable film; receiving reflected light of the light-permeable film in a regular reflection direction by a light receiving element, acquiring a reflected light intensity signal that is a time-series signal of a signal intensity of the reflected light until the end of evaporation of the light-permeable film, the reflected light intensity signal indicating a change of the signal intensity, and saving the acquired reflected light intensity signal into a storage device; estimating the signal intensity of the reflected light that is obtained by irradiating the light-permeable film with light from the light source, based on a spectroscopic intensity of the light source, a spectroscopic sensitivity of the light receiving element, and an optical constant of the light-permeable film, and acquiring an estimation signal indicating a change of an estimated value of the signal intensity of the reflected light as a film thickness of the light-permeable film changes; specifying a time range where a desired time point exists, based on the number of peaks in a signal waveform of the reflected light intensity signal from a base point as a time point of the end of evaporation of the light-permeable film to the desired time point; coordinating a signal waveform of the estimation signal and the signal waveform of the reflected light intensity signal with each other such that a time point when the film thickness is equal to zero in the signal waveform of the estimation signal and the base point in the signal waveform of the reflected light intensity signal coincide with each other, and specifying a film thickness range corresponding to the time range in the signal waveform of the estimation signal, based on the number of peaks in the signal waveform of the estimation signal; and acquiring a signal intensity of the reflected light at the desired time point, and setting a film thickness corresponding to that estimated value of the signal intensity of the reflected light which corresponds to a film thickness falling within the film thickness range and coincides with the signal intensity of the reflected light at the desired time point, as a film thickness of the light-permeable film at the desired time point. arranging a volatile light-permeable film to be measured on a substrate, and irradiating the light-permeable film with light from a light source until an end of evaporation of the light-permeable film; receiving reflected light of the light-permeable film in a regular reflection direction by a light receiving element, acquiring a reflected light intensity signal that is a time-series signal of a signal intensity of the reflected light until the end of evaporation of the light-permeable film, the reflected light intensity signal indicating a change of the signal intensity, and saving the acquired reflected light intensity signal into a storage device; estimating the signal intensity of the reflected light that is obtained by irradiating the light-permeable film with light from the light source, based on a spectroscopic intensity of the light source, a spectroscopic sensitivity of the light receiving element, and an optical constant of the light-permeable film, and acquiring an estimation signal indicating a change of an estimated value of the signal intensity of the reflected light as a film thickness of the light-permeable film changes; specifying a time range where a desired time point exists, based on the number of peaks in a signal waveform of the reflected light intensity signal from a base point as a time point of the end of evaporation of the light-permeable film to the desired time point; coordinating a signal waveform of the estimation signal and the signal waveform of the reflected light intensity signal with each other such that a time point when the film thickness is equal to zero in the signal waveform of the estimation signal and the base point in the signal waveform of the reflected light intensity signal coincide with each other, and specifying a film thickness range corresponding to the time range in the signal waveform of the estimation signal, based on the number of peaks in the signal waveform of the estimation signal; and acquiring a signal intensity of the reflected light at the desired time point, and setting a film thickness corresponding to that estimated value of the signal intensity of the reflected light which corresponds to a film thickness falling within the film thickness range and coincides with the signal intensity of the reflected light at the desired time point, as a film thickness of the light-permeable film at the desired time point.