To provide a washer for detecting the slack of a versatile bolt that can detect the slack even for the structure that the detecting work in the field is difficult and the structure that the tremendous time for the detecting work, etc. is required:to dissolve such problem as lowering the strength, restricting the shape and increasing the cost for the bolt against the slack of the bolt tightening body.

The one that puts a piezoelectric element (1b) to a washer base material (1a) is made to be a washer (1) for detecting the bolt slack and the washer (1) for detecting the bolt slack can be vibrated in the thickness direction by making the piezoelectric element (1b) expand and contract in the longitudinal direction, and it constitutes so that the vibration condition of the washer (1) for detecting the bolt slack can be detected by the piezoelectric element (1b) itself. The change in the natural vibration frequency of the washer (1) for detecting the bolt slack is detected and the slack of the bolt tightening body is detected by comparing the natural vibration frequency of the washer (1) for detecting the bolt slack in the initial tightening condition with the slack condition of the bolt tightening body.