PURPOSE: To improve the stability and the profitability by a simple structure, by providing a clad layer of a specific material on the outside peripheral part of a core of an optical fiber, and also, providing a covered layer consisting of plastic of a specific material on its outside peripheral part.

CONSTITUTION: Pure quartz glass (1.458 refractive index) is brought to wire drawing so that the outside diameter becomes 200μm, as a core 1, and thereafter, immediately its outside peripheral part is coated and covered with a vinylidene fluoride copolymer (120°C melting pint, 1.42 refractive index) as a clad layer 2 so that the outside diameter becomes 230μm. At the time of extruding the clad layer 2, or at the time of hardening the application, a tensile stress is allowed to remain behind in the clad layer 2 at the time when the formation has been completed by applying the stress in the longitudinal direction. Also, to its outside peripheral part, a composite (1.54 refractive index) mainly composed of urethane acrylate is coated as a covered layer 3 until the outside diameter becomes 500μm, and brought to a crosslinking by an ultraviolet ray lamp. In such a way, the noise resistance and the stability are excellent, a microbend loss is scarcely generated, a malfunction is prevented, and also, metallic fittings, etc. are not required, therefore, the structure is simple, and also, the installation space is small, and this sensor is obtained at a low cost.