A wheel bearing apparatus comprising: an outer member integrally formed on its inner circumference with double row outer raceway surfaces (2a, 2a); an inner member including a hub wheel and at least one inner ring, the hub wheel being integrally formed on its one end with a wheel mounting flange and having a cylindrical portion axially extending from the wheel mounting flange, the inner ring being press-fitted onto the cylindrical portion of the hub wheel, the inner member being formed on its outer circumference with double row inner raceway surfaces (4a, 5a) opposing to the double row outer raceway surfaces (2a, 2a); double row rolling elements (3, 3) contained between the inner raceway surfaces (4a, 5a) and outer raceway surfaces (2a, 2a) of the inner member and outer member; a pulser ring adapted to be fitted onto the outer circumference of the inner ring and having characteristics alternately and equidistantly varying along its circumferential direction; a cup-shaped second cap (16, 16', 29, 40, 47, 49, 52) press-formed of steel sheet and fitted on the inboard-side end of the outer member; a rotational speed sensor mounted on the second cap (16, 16', 29, 40, 47, 49, 52) at a radially outer position thereof, the rotational speed sensor being arranged oppositely to the pulser ring via a predetermined axial air gap; a cup-shaped first cap (15, 39, 46, 48) being press-fitted into the inner circumference of an inboard-side end of the outer member via a predetermined interference, being press-formed of non-magnetic austenitic stainless steel sheet and comprising a cylindrical fitting portion (15a, 39a, 46a, 48a) press-fitted into the inner circumference of the inboard-side end of the outer member, a disc portion (15c, 39b) extending radially inward from the fitting portion (15a, 39a, 46a, 48a) and opposing to the pulser ring via a small axial gap; the rotational speed sensor being arranged oppositely to the pulser ring via the first cap (15, 39, 46, 48) with being abutted against or closed to the disc portion (15c, 39b) of the first cap (15, 39, 46, 48); and the second cap (16, 16', 29, 40, 47, 49, 52) being press-fitted into the inner circumference of an inboard-side end of the outer member via a predetermined interference; characterized in that a bulged portion is formed on a bottom portion (16c, 40c) of the second cap (16, 16', 29, 40, 47, 49, 52) at a region nearer to the ground and that a drain (26, 30) which is radially cut-through is formed in a bottom wall of the bulged portion. an outer member integrally formed on its inner circumference with double row outer raceway surfaces (2a, 2a); an inner member including a hub wheel and at least one inner ring, the hub wheel being integrally formed on its one end with a wheel mounting flange and having a cylindrical portion axially extending from the wheel mounting flange, the inner ring being press-fitted onto the cylindrical portion of the hub wheel, the inner member being formed on its outer circumference with double row inner raceway surfaces (4a, 5a) opposing to the double row outer raceway surfaces (2a, 2a); double row rolling elements (3, 3) contained between the inner raceway surfaces (4a, 5a) and outer raceway surfaces (2a, 2a) of the inner member and outer member; a pulser ring adapted to be fitted onto the outer circumference of the inner ring and having characteristics alternately and equidistantly varying along its circumferential direction; a cup-shaped second cap (16, 16', 29, 40, 47, 49, 52) press-formed of steel sheet and fitted on the inboard-side end of the outer member; a rotational speed sensor mounted on the second cap (16, 16', 29, 40, 47, 49, 52) at a radially outer position thereof, the rotational speed sensor being arranged oppositely to the pulser ring via a predetermined axial air gap; a cup-shaped first cap (15, 39, 46, 48) being press-fitted into the inner circumference of an inboard-side end of the outer member via a predetermined interference, being press-formed of non-magnetic austenitic stainless steel sheet and comprising a cylindrical fitting portion (15a, 39a, 46a, 48a) press-fitted into the inner circumference of the inboard-side end of the outer member, a disc portion (15c, 39b) extending radially inward from the fitting portion (15a, 39a, 46a, 48a) and opposing to the pulser ring via a small axial gap; the rotational speed sensor being arranged oppositely to the pulser ring via the first cap (15, 39, 46, 48) with being abutted against or closed to the disc portion (15c, 39b) of the first cap (15, 39, 46, 48); and the second cap (16, 16', 29, 40, 47, 49, 52) being press-fitted into the inner circumference of an inboard-side end of the outer member via a predetermined interference; characterized in that a bulged portion is formed on a bottom portion (16c, 40c) of the second cap (16, 16', 29, 40, 47, 49, 52) at a region nearer to the ground and that a drain (26, 30) which is radially cut-through is formed in a bottom wall of the bulged portion.