To easily and visually check, from a remote position, a loosened state of a fastening means that fastens a member by screwing.

A looseness determining tool 20 is provided for visually checking a loosened state of a bolt 11 and nuts 12a, 12b, and the tool includes: a washer 21, which is axially pressed by screwing the bolt 11 and nuts 12a, 12b and has a retention groove 21a extending in a circumference direction on an outer circumference face; and a looseness determining member 22 pinched in the retention groove 21a by axial elastic deformation of the washer 21 by a prescribed fastening torque. If the fastening torque decreases to a prescribed value or lower, the looseness determining member 22 drops by its own weight from the retention groove 21a.