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.