This paper shows a basic study for the health monitoring of the bridge bearing that is one of important structural components of the bridge. A Brillouin-scattering based fiber-optic sensor; PPP-BOTDA, was adopted to assess the strain distribution changes of girders due to uncertain changes of the bridge bearing performance; i.e., boundary condition changes. An experimental verification was carried out using a simply-supported beam structure with the bearings that could change the performance of their slide and rotation mechanisms. We then proposed an index to extract the change of strain distribution profile due to the change of bearing performance with considering the measurement noise. It was additionary shown that the index was able to indicate the gradual changes of bridge bearing performance quantitatively from the result of an analytical verification.