An inner diameter-side cylindrical member 37 of a supporting ring 36 is externally fitted on a shoulder 35 of an inner ring 3 constituting a hub. An encoder 29a is supported on the outer peripheral surface of an outer diameter-side cylindrical portion 38 constituting the supporting ring 36. Further, to a metal core 30a internally fitted to an outer ring 1 is supported and fixed a rotation-detecting sensor 21b, and also a proximal end portion of a seal member 44 is connected and fixed thereto. An edge of at least one of seal lips 45a, 45b, 45c of the seal member 44 is brought to be in sliding contact with the supporting ring 36 around the entire circumference. Since the rotation-detecting sensor 21b and the seal member 44 are supported by the core metal 30a, a rolling bearing unit can be downsized. Further, since the encoder 29a is placed on the outer peripheral surface of the outer diameter-side cylindrical portion 38, the width dimension of a surface with which the edge of each of the seal lips 45a, 45b, 45c is brought to be in sliding contact is secured for excellent sealing ability. The structure provides a rolling bearing unit with a rotating speed-detecting device, where the bearing unit is downsized and has a construction capable of achieving excellent sealing ability.