An elastic body 35 of a seal member 30 for sealing off a space between an outer race 26 and an inner race 27 of a bearing 15 has three seal lips 35a to 35c and an extending portion 35d. By allowing the extending portion 35 to be held in place in a gap null between an axial end face of the outer race 26 of the bearing 15 and an axial end face of a bearing support hole in a knuckle 13, the penetration of water into the gap null can be prevented by making use of the existing seal member while avoiding increases in the numbers of components to be assembled and man-hours needed to assemble the components by obviating the necessity of any special seal member, whereby the generation of rust resulting from electrolytic corrosion can be prevented effectively.