To make it possible to detect a water leakage accident quickly and surely in high accuracy with a compact, light-weight, simple, inexpensive structure by projecting light on a reflecting plate arranged on the fluid surface of a leaking water, and receiving the light reflected from the upper surface of the plate.

Infrared rays emitted from a light emitting element 5 is reflected from the upper surface of a reflecting plate 6 and received by a photodetector 7. Since the reflecting plate 6 is arranged on the fluid surface of leaking water, the generated leaking water sticks to the upper surface of the reflecting plate 6. The light emitted from the element 5 is absorbed and irregularly reflected at the same time. Thus, the amount of the received light in the photodetector 7 is decreased. Therefore, the occurrence of the leakage can be detected by the decrease in amount of the received light in the photodetector 7. In this way, even if the amount of the leakage is less, the leakage can be detected quickly and highly sensitively, and the countermeasure for accident can be performed in an early stage. Since mechanical processing is not required for the set place, the set place can be readily selected. Since the number of parts required for the constitution is small and it is not necessary to perform special control, the compact and light-weight detector can be constituted simply at a low cost.