It is possible to obtain best performance in a structure in which moment rigidity can be improved by setting a pitch circle diameter of an outboard row to be larger than a pitch circle diameter of an inboard row.Thermally-hardened layers 16a, 16b are formed, at two locations along an axial direction, in portions of an inner circumferential surface of an outer ring 3 including outer ring raceways 12a, 12b. The thermally-hardened layers 16a, 16b are separated from each other by an unhardened portion 17 in an axially middle portion of the outer ring 3. A non-cylindrical curved portion 18 having an arcuate generatrix is provided on the axially middle portion of the inner circumferential surface of the outer ring 3, between the outer ring raceways 12a, 12b. A thickness Ta of the thermally-hardened layer 16a in the portion including the outer ring raceway 12a on an axially outer side is larger than a thickness Tb of the thermally-hardened layer 16b in the portion including the outer ring raceway 12b on the axially inner side.