An optical fiber sensor connected to an optical fiber communication line usable on an optical fiber communication line of the Internet etc. and able to share a communication line and a sensor line is provided. It has an optical fiber sensor line being an optical fiber (11d, 11b, 11e) provided with a core and with a cladding provided on an outer circumference of the core and provided so as to be optically coupled with the optical fiber communication line (10, 11a), which has a sensor portion (SPa, SPb, SPc, SPd) for enabling interaction of a portion of transmitted light with an external environment and includes a portion 11b for transmitting at least communication light from the optical fiber communication line; a light source (17) emitting sensor light to an incident end of the optical fiber sensor line; and a light receiving portion (18) detecting the sensor light emitted from an emitting end of the optical fiber sensor line via the sensor portion.

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