A wheel bearing apparatus comprising: an axle housing into which a driving shaft (D/S) connecting to a differential gear is inserted and which is supported on the lower surface of a vehicle body; a wheel hub which is joined to the driving shaft (D/S) and integrally has a wheel mounting flange for attaching a wheel and in which a cylindrical portion extending in an outer circumference in an axial direction is formed; a wheel bearing which is fitted between the cylindrical portion of the wheel hub and an opening of the axle housing and rotatably supports the wheel; an outer member in which the wheel bearing integrally has a body mounting flange to be attached to the axle housing in the outer circumference and a pilot portion fitted into the axle housing at an end portion of an inner side, and double row outer raceway surfaces (4a, 4a) are integrally formed in the inner circumference; an inner member which is press-fitted in the cylindrical portion of the wheel hub and configured of at least one inner ring (10, 10) in which inner raceway surfaces (10a, 10a) are formed opposite to the double row outer raceway surfaces (4a, 4a); double row rolling bodies (5, 5) which are rollably accommodated between the inner member and both raceway surfaces of the outer member via a cage; and a seal (12,12) which is mounted in an opening of an annular space formed between the outer member and the inner member, and wherein an annular groove is formed at the pilot portion of the outer member, a seal ring formed of a synthetic rubber is mounted in the annular groove and elastically contacts the axle housing and then a slight gap of a fitting portion between the outer member and the axle housing is blocked off, characterized in that the pilot portion of the outer member is configured of a cylindrical portion extending from the body mounting flange in the axial direction and a tapered portion gradually decreasing in its diameter from the cylindrical portion to the end surface of the inner side, that the fitting portion of the axle housing is formed corresponding to the shape of the pilot portion of the outer member, that the annular groove is formed at the tapered portion of the pilot portion and that an inner diameter of the seal ring unit is set to be smaller than a groove bottom diameter of the annular groove. an axle housing into which a driving shaft (D/S) connecting to a differential gear is inserted and which is supported on the lower surface of a vehicle body; a wheel hub which is joined to the driving shaft (D/S) and integrally has a wheel mounting flange for attaching a wheel and in which a cylindrical portion extending in an outer circumference in an axial direction is formed; a wheel bearing which is fitted between the cylindrical portion of the wheel hub and an opening of the axle housing and rotatably supports the wheel; an outer member in which the wheel bearing integrally has a body mounting flange to be attached to the axle housing in the outer circumference and a pilot portion fitted into the axle housing at an end portion of an inner side, and double row outer raceway surfaces (4a, 4a) are integrally formed in the inner circumference; an inner member which is press-fitted in the cylindrical portion of the wheel hub and configured of at least one inner ring (10, 10) in which inner raceway surfaces (10a, 10a) are formed opposite to the double row outer raceway surfaces (4a, 4a); double row rolling bodies (5, 5) which are rollably accommodated between the inner member and both raceway surfaces of the outer member via a cage; and a seal (12,12) which is mounted in an opening of an annular space formed between the outer member and the inner member, and wherein an annular groove is formed at the pilot portion of the outer member, a seal ring formed of a synthetic rubber is mounted in the annular groove and elastically contacts the axle housing and then a slight gap of a fitting portion between the outer member and the axle housing is blocked off, characterized in that the pilot portion of the outer member is configured of a cylindrical portion extending from the body mounting flange in the axial direction and a tapered portion gradually decreasing in its diameter from the cylindrical portion to the end surface of the inner side, that the fitting portion of the axle housing is formed corresponding to the shape of the pilot portion of the outer member, that the annular groove is formed at the tapered portion of the pilot portion and that an inner diameter of the seal ring unit is set to be smaller than a groove bottom diameter of the annular groove.