Liquid leakage detection system

A liquid leakage detection system comprising a plurality of light guides each being provided with a relatively straight light guide core that has a predetermined index of refraction and a cladding which is an outer layer wholly surrounding said light guide core, said cladding having an index of refraction lower than that of said light guide core and is adapated to be swelled by a liquid whose leakage is to be detected, said light guides being stranded into a detector cable and being provided with a covering sheath, the ends of said light guide core being uncoated so that a light ray entering into said light guide core at one end thereof may emerge from the other end thereof with a minimum transmission loss, a light-emitting element optically coupled to one end of said detector cable, an optical sensor optically coupled to the other end of said detector cable, and an electrical circuit for deriving the output signal whose level or intensity is in proportion to the intensity of the light rays intercepted by said optical sensor whereby the liquid leakage may be detected in terms of the drop in the intensity of the light rays intercepted by said optical sensor due to the adhesion of said liquid to said cladding.