This article discusses an application of optical fiber sensing to observation or diagnosis of structural facilities in ports or coastal areas. A BOTDR (Brillouin Optical-Fiber Time Domain REflectometer) has been applied to real time observation of sand leakage from the inside of caissons on trial. Experiments with a small model of cassion have been conducted for verifying whether the BOTDR is a realistic measure for such a need. In the experiments three types of the fiber sensor were employed. This parer includes the principle of the BOTDR, experimental descriptions as well as theoretical analysis on the long-term reliability of the optical fiber sennsor.