To provide a strain sensor and a bolt having the strain sensor capable of catching stress change with high sensitivity by shortening the distance from a stress concentrated point, embedding a stain gauge in an arbitrary depth according to bolt cutting work, arbitrarily selecting a sealing material of the strain gauge, easily being produced by a photo application, lowering the cost and making it accurate.

In the bolt 1, the loosening of bolt tightness can be detected by attaching the strain sensor 2, and the strain gauge 2 having a gauge resistor 21 extending or shrinking by receiving a stress in the axial direction of the bolt is provided in a recess 13 formed in the bolt head 12. The strain gauge 2 is provided in a gap 14 which is formed in the recess 13.