To provide a low cost strain sensor capable of directly sensing the position of a strain measuring point without necessity of temperature correction, and to provide a strain sensing unit.

The strain sensor is provided with an optical fiber in which grating is written in a core, and a sensing section for including the grating. The sensing section is formed so as to transmit an external force applied thereto to the grating. The sensor changes its reflecting light amount when the force is applied to the grating. The strain sensing unit displays distance-reflecting light amount characteristics including a reflecting waveform in each grading on an OTDR by connecting the sensor having a plurality of the gratings having different reflecting beam wavelengths to the OTDR having a wide band light source including the reflecting peak wavelength.