To solve problems in a conventional water invasion detecting method for a cable that a water-absorptive expanding agent or an oil-absorptive expanding agent has to be included in the cable, that the outer diameter of the cable is necessarily thickened since space has to be secured therein for including the water-absorptive expanding agent or the oil-absorptive expanding agent, and that cost is increased by the amount of the expanding agents.

A light pulse is launched into optical fiber in a cable of every kind such as an optical cable or a power cable. Based on the ratio of stroke side intensity to anti-stroke side intensity of resultantly generated Raman scattered light, whether the cable is invaded by water or not and/or what part is invaded by water are/is detected.