Bolt sensor

1. A bolt sensor configured to detect a fastened state of a bolt, comprising:

a sensor body including a through hole into which a shaft of the bolt is to be inserted;

a light guide extending along an outer periphery of the sensor body; and

a light source configured to emit light to the light guide based on output of the sensor body, wherein

the light guide includes a light guide plate having an incident surface through which the light from the light source enters into the light guide plate and an emission surface from which the light entered into the light guide plate through the incident surface is emitted, and

an opposing surface, of the light guide plate, on an opposite side of the emission surface has a plurality of concaves which reflect the light entered into the light guide plate through the incident surface toward the emission surface.

2. The bolt sensor according to claim 1 , wherein a density of the plurality of concaves are higher in a part of the opposing surface than other parts of the opposing surface.

3. The bolt sensor according to claim 1 , wherein the light guide further includes a prism sheet disposed on the emission surface of the light guide plate and a diffusion sheet disposed on the prism sheet.

4. The bolt sensor according to claim 1 , wherein the light guide extends to surround the outer periphery of the sensor body substantially entirely.

5. The bolt sensor according to claim 1 , further comprising a cover covering the light guide.

6. The bolt sensor according to claim 1 , further comprising a board which is connected to the sensor body and the light source and which is configured to turn on the light source based on a comparison between the output of the sensor body and a predetermined value.

7. The bolt sensor according to claim 1 , wherein the sensor body includes a body section including the through hole and a strain sensor attached to the body section.

8. The bolt sensor according to claim 1 , wherein the light source is provided on the sensor body.