PURPOSE: To aim at preventing a wheel from loosening and being stolen, by providing such an arrangement that an internal gear ring having ratchet teeth is integrally incorporated with the inner surface of the end of a threaded cylinder opening, and a lock body having a locking plate is assembled to the end plate of a cap nut so that the locking plate and the ratchet teeth of the internal gear ring may be engaged with and disengaged from each other.

CONSTITUTION: When a lock body N is unlocked by a key which is not shown, a mandrel 47 is rotated and the large diameter section of a cam 34 pushes one side of a recess 33 which is remote from ratchet teeth 32, to retract a locking plate 31 so that the engagement between the ratchet teeth 32 and the ratchet teeth 18 of an internal gear ring 17 is released, and therefore, a cap nut N is allowed to be rotated, thereby a wheel may be removed. When the key is turned in its locking direction after the cap nut N is tightened. The large diameter section of the cam 34 urges the locking plate 31 to project so that the ratchet teeth 32 are engaged with the ratchet teeth 18 to inhibit the cap nut N from rotating. Thereby, the loosening of the wheel and the theft thereof is aimed at being prevented.