Abstract: An improved apparatus suitable for detecting the outflow of a liquid such as petroleum or the like and petrochemical products from a storage tank or pipe line thereof is disclosed. The detecting apparatus comprises a detecting element including a light transmitting path made of a transparent solid material having a refractive index larger than the refractive index of water through which the light is transmitted from a light source to a light detector. If the peripheral wall surface of the light transmitting path is wetted with a liquid to be detected which has a larger refractive index than the material forming the light transmitting path, the light being transmitted through the path leaks out of the path at the wetted area of the peripheral wall surface due to the difference of the refractive index between the liquid and the path.