To provide a whirl stop device capable of maintaining surely a whirl stop state for a tightening bolt of a tightening device.

This whirl stop device has a recessed groove 8 arranged coaxially with respect to an insertion through-hole 6 in a one-side article to surround the insertion through-hole 6, and equipped with an irregular part 9 arrayed at least circumferentially and directionally on an outside inner circumferential wall 8". A locking hole 10 long along a circumferential direction is formed to be engaged with an elastic locking piece 12a of a loosening-preventive cap 12 engaged with the recessed groove 8 in a position opposed via the insertion through-hole 6 at a bottom wall 8''' of the recessed groove 8, and a locking stage 11 is formed to be locked with a tip locking part 12a' of the elastic locking piece 12a at an outer circumferential edge of the locking hole 10. The loosening-preventive cap 12 comprises a bottomed cylindrical body projected oppositely with the elastic locking pieces 12a, 12a at an opened end side is constituted to form a circumferential face into an irregular surface 13 meshed with the irregular part 9, and has a square recessed part 14 engaged so as to coincide with a square head 3a of the bolt 3 through the opened end on a deep wall face.