The radius of curvature r1, r2 of the cross-sectional shape of an inner ring raceway lid is made smaller at a central portion 19 and larger at end portions 20. A contact ellipse at the end portions is made small by increasing the radius of curvature r2 of the end portions 20 to thereby secure an allowable moment load. In contrast with this, an angular gap is reduced by reducing the radius of curvature r1 of the central portion 19.