In Japan, many people live in mountainous area and sometimes they experienced landslides under heavy rain. Also there are many superannuated constructions which built at 1960-70's. Wide area monitoring systems which can detect critical conditions are useful to prevent disasters. This study proposes new fiber optic loss variation measurement system which can use for wide area installed branched fiber networks. It can detect local strain variation in combination with appropriate sensors. This paper proposes new fiber optic sensing systems using different length fiber branches. If some transmission loss was induced anywhere on branched fibers, the Rayleigh backscattering signal response would be changed. We can detect where and how much the loss induced. The simulation and experiments have done using 2 branched fiber networks. The experimental results using OTDR (Optical Time Domain Reflectometer) have good agreements with simulation results. We can calculate the loss value and its position backward from the Rayleigh back scattering signal response.