When an inner ring of a driving-wheel supporting hub unit is fixed to a hub, a pressing die is displaced downward and an annular working surface at a lower end surface of the pressing die is pressed against a front end of a cylindrical portion over its whole periphery. Further, in this state, the pressing die is pressed downward toward the hub without rocking the pressing die relative to the hub. Thus, the cylindrical portion is plastically deformed radially outward along the working surface simultaneously over its whole periphery. By adopting this method, the operation of plastically deforming the cylindrical portion at the inner end of the hub to form a caulking portion, can be easily and performed accurately. Accordingly, the structure of the caulking portion forming apparatus can be simplified.