A hub unit bearing H1 includes a rotational speed detector 9 which is made up of an encoder 6 and a sensor 8. A seal 5b on a side facing a center side of a vehicle includes a primary seal 5bp and a secondary seal 5bq which are disposed in that order in an axial direction from rolling elements 32 on a side facing the center side of the vehicle towards the center side of the vehicle so as to define a primary sealed space portion I between the vehicle's center side facing rolling elements 32 and the primary seal 5bp and a secondary sealed space portion II between the primary seal 5bp and the secondary seal 5bq, with the rotational speed detector 9 disposed within the secondary sealed space portion II.