A wheel supporting system comprising: a hub shaft of a wheel hub to which a wheel is to be attached; and an outer ring of a constant velocity joint which is coupled to the hub shaft in such a manner that torque can be transmitted therebetween, wherein a radially inner side annular portion and a radially outer side annular portion are provided both on an end face of the hub shaft and an end face of the outer ring of the constant velocity joint which is brought into abutment with the end face of the hub shaft, and wherein side face splines (71, 76) are formed individually on either annular portions of the radially inner side annular portions and the radially outer side annular portions in such a manner as to mesh with each other so that the wheel hub and the outer ring of the constant velocity joint can be coupled together in such a manner that torque can be transmitted between the wheel hub and the constant velocity joint, characterized in that flat surfaces (73, 78) are formed individually on the other annular portions in such a manner as to be brought into close contact with each other so as to form an annular seal portion, wherein the side face splines (71, 76) are formed individually on the radially inner side annular portions of the end face of the hub shaft and the end face of the outer ring of the constant velocity joint, and the flat surfaces (73, 78) portions are formed individually on the radially outer side annular portions in such a manner as to form the annular seal portion, wherein the radially inner side annular portion on the end face of the hub shaft is formed into a step-like configuration by being recessed further than the radially outer side annular potion thereof to an extent corresponding to a step portion, and the radially inner side annular portion on the end face of the outer ring of the constant velocity joint is formed into a step-like configuration by being caused to project further than a radially outer side annular portion thereof to an extent corresponding to a step portion. a hub shaft of a wheel hub to which a wheel is to be attached; and an outer ring of a constant velocity joint which is coupled to the hub shaft in such a manner that torque can be transmitted therebetween, wherein a radially inner side annular portion and a radially outer side annular portion are provided both on an end face of the hub shaft and an end face of the outer ring of the constant velocity joint which is brought into abutment with the end face of the hub shaft, and wherein side face splines (71, 76) are formed individually on either annular portions of the radially inner side annular portions and the radially outer side annular portions in such a manner as to mesh with each other so that the wheel hub and the outer ring of the constant velocity joint can be coupled together in such a manner that torque can be transmitted between the wheel hub and the constant velocity joint, characterized in that flat surfaces (73, 78) are formed individually on the other annular portions in such a manner as to be brought into close contact with each other so as to form an annular seal portion, wherein the side face splines (71, 76) are formed individually on the radially inner side annular portions of the end face of the hub shaft and the end face of the outer ring of the constant velocity joint, and the flat surfaces (73, 78) portions are formed individually on the radially outer side annular portions in such a manner as to form the annular seal portion, wherein the radially inner side annular portion on the end face of the hub shaft is formed into a step-like configuration by being recessed further than the radially outer side annular potion thereof to an extent corresponding to a step portion, and the radially inner side annular portion on the end face of the outer ring of the constant velocity joint is formed into a step-like configuration by being caused to project further than a radially outer side annular portion thereof to an extent corresponding to a step portion.