A rim drop section dented in a shape of a ring is provided on an inner side of a bead seat section of a rim for holding a bead section of a tire. Furthermore, a recessed section is provided on this rim drop section, and a detecting device main body of a tire air pressure detecting device is accommodated in this recessed section. When a tire is assembled on a cast wheel by utilizing a tire changer or the like, or when the tire is removed from the cast wheel, the bead sections of the tire are prevented from interfering with an air pressure sensor, and damaging of the air pressure sensor is prevented.