A new type of fiber sensor using OTDR (Optical Time-Domain Reflectometer) has been developed whose concept is intended for structural damage monitoring through fiber networks spread in wide areas to be monitored. The sensor has a spliced hetero-core structure in which a short-length, different core-size fiber is inserted in a single-mode-fiber network line with low transmission loss for the wavelength of 1.3μm. In this study, two types of sensor are demonstrated which are low-insertion-loss and high-sensitivity ones. The variation of the bending losses has been experimentally measured to be 9x10^-2, and 3.6dB/deg for the low-insertion loss and the high-sensitivity types, respectively, as a function of rotation angles with which the hetero-core portion is wound. Qualitative discussions are also made both on how these two sensors show high sensitivities to relatively large-radius-bending of the hetero-core portion and on why their characteristics performances are appeared in different fashions