The experiment introduced in this paper was performed in order to measure humidity during snow crystal formation by using the FINEDEW chilled mirror hygrometer. FINEDEW is a micro chilled mirror hygrometer employing a pair of thin fiber optics, which enables the real-time measurement of dew point temperatures by keeping the mirror close to the fiber line that generates the snow crystals. The results of this experiment have clarified that it is possible to measure dew point temperatures between -5℃ and -40℃ by this device. In comparison with the previous report by Nakaya (1954), the humidity measured by the FINEDEW is a few percent lower than his report. We propose that the difference is caused by the fact that we measure water vapor in the air, while Nakaya (1954) measured water vapor and liquid water droplets in the air. This paper also presents the principles and features of the humidity measuring system employing FINEDEW.