To prevent foreign matter from entering a periphery part of a sensor of a rotation speed detector.

In a cover body 17 of the rotation speed detector 100, a circumferential wall part 26 is extended from one end part of a cylindrical main body part 23 in the axial direction to the inside in the radial direction, the tip part thereof is along an outer circumferential surface of a straight outer circumferential part 4a of a constant-velocity joint and bent at substantially right angles to a side of a vehicle inner to form a first bent part 27, the tip part of the first bent part 27 is along an outer circumferential surface of a taper outer circumferential part 4b of the constant-velocity joint and bent to the side of the vehicle inner to form a second bent part 28. The outer circumferential surface of the constant-velocity joint is covered by the first and second bent parts 27, 28 via a micro gap (labyrinth gap). The foreign matter flowing down along the outer circumferential surface of the constant-velocity joint is prevented from entering the periphery part of the head part 18 of the sensor 16 by each gap.