A transmission loss of each dried and water-saturated plastic optical fiber (POF) is measured. In range from 500 nm to 720 nm and from 785 nm to 810 nm, the transmission loss P1 of the dried POF and a difference of the transmission loss from the water-saturated POF to the dried POF satisfy at least one of the conditions, 0L/P11, 0L100 and 10L+P1200. Further, in the wavelength range from the ultraviolet ray to infrared ray, the absorption wavelength of OH vibration in water contained in the POF and the absorption wavelength X of harmonic absorptions of atomic vibrations are detected, and the above conditions are the wavelength ranges except the absorption wavelength X. In the performance of optical communication in these wavelength ranges, the transmission loss does not increase so much even when the the POF absorbs water. Therefore the adequate optical communication can be performed stably.