A wheel speed detector for detecting a relative rotating speed between an outer ring and an inner ring by means of a magnetic sensor in association with a magnetic ring. A stationary seal member is fixed to the stationary ring and a rotatable seal member is fixed to the rotatable ring. The seal members engage to seal a gap between the inner ring and the outer ring. The magnetic sensor is fixed to a radial inner surface of the stationary seal member and the magnetic ring is fixed to a radial outer or a radial inner surface of the rotatable seal member and surfaces of the magnetic ring other than that fixed to the radial inner surface of the rotatable seal member are covered with a seal lip.