A rolling bearing unit, comprising: a stationary bearing ring that is supported by and fastened to a support member that does not rotate during use; a rotating bearing ring that is concentrically arranged with the stationary bearing ring; stationary-side raceway surfaces and rotating-side raceway surfaces that are formed around circumferential surfaces of the stationary bearing ring and rotating bearing ring that face each other; a plurality of rolling elements that are located between the stationary-side raceway surfaces and rotating-side raceway surfaces so as to be able to roll freely; and a seal member that blocks the rolling-element installation space where the rolling elements are located from the external space; the rotating bearing ring comprising: a rotating-side main body that has a cylindrical member that is made using a synthetic resin containing reinforced fibers, or an aluminum alloy; a rotating-side core that has a cylindrical member that is made using a ferrous alloy, part thereof being molded to the rotating-side main body, and a portion on the stationary bearing ring side of the circumferential surface thereof being exposed from the rotating -side main body; and a rotating-side raceway member that has a cylindrical member that is made using a ferrous alloy, provided with the rotating-side raceway surfaces formed around the circumferential surface on the stationary bearing ring side thereof, and that is fitted with and fastened to the portion on the stationary bearing ring side of the circumferential surface of the rotating-side core that is exposed from the rotating-side main body; and the stationary bearing ring comprising: a stationary-side main body that has a cylindrical member that is made using a synthetic resin containing reinforced fibers, or an aluminum alloy; a stationary-side core that has a cylindrical member that is made using a ferrous alloy, part thereof being molded to the stationary-side main body, and a portion on the rotating bearing ring side of the circumferential surface thereof being exposed from the stationary-side main body; and a stationary-side raceway member that has a cylindrical member that is made using a ferrous alloy, provided with the stationary-side raceway surfaces formed around the circumferential surface on the rotating bearing ring side thereof, and that is fitted with and fastened to the portion on the rotating bearing ring side of the circumferential surface of the stationary-side core that is exposed from the stationary-side main body. a stationary bearing ring that is supported by and fastened to a support member that does not rotate during use; a rotating bearing ring that is concentrically arranged with the stationary bearing ring; stationary-side raceway surfaces and rotating-side raceway surfaces that are formed around circumferential surfaces of the stationary bearing ring and rotating bearing ring that face each other; a plurality of rolling elements that are located between the stationary-side raceway surfaces and rotating-side raceway surfaces so as to be able to roll freely; and a seal member that blocks the rolling-element installation space where the rolling elements are located from the external space; a rotating-side main body that has a cylindrical member that is made using a synthetic resin containing reinforced fibers, or an aluminum alloy; a rotating-side core that has a cylindrical member that is made using a ferrous alloy, part thereof being molded to the rotating-side main body, and a portion on the stationary bearing ring side of the circumferential surface thereof being exposed from the rotating -side main body; and a rotating-side raceway member that has a cylindrical member that is made using a ferrous alloy, provided with the rotating-side raceway surfaces formed around the circumferential surface on the stationary bearing ring side thereof, and that is fitted with and fastened to the portion on the stationary bearing ring side of the circumferential surface of the rotating-side core that is exposed from the rotating-side main body; and a stationary-side main body that has a cylindrical member that is made using a synthetic resin containing reinforced fibers, or an aluminum alloy; a stationary-side core that has a cylindrical member that is made using a ferrous alloy, part thereof being molded to the stationary-side main body, and a portion on the rotating bearing ring side of the circumferential surface thereof being exposed from the stationary-side main body; and a stationary-side raceway member that has a cylindrical member that is made using a ferrous alloy, provided with the stationary-side raceway surfaces formed around the circumferential surface on the rotating bearing ring side thereof, and that is fitted with and fastened to the portion on the rotating bearing ring side of the circumferential surface of the stationary-side core that is exposed from the stationary-side main body.