A first step of measuring a change over time in the spectral transmission spectrum of a measurement sample by its exposure to the light of a light source including an ultraviolet radiation for a preset light exposure time, a second step of performing a correction according to the change over time in the spectral transmission spectrum of the measurement sample based on the result of the measurement obtained by the first step, and a third step of calculating the final in vitro predicted SPF of the measurement sample using the result of the correction obtained by the second step are included.