A first portion of a perimeter of an outer circumferential portion of a tire is magnetized with a first polarity along the steel belt in one circumferential direction less than 360° and a second portion of the perimeter is magnetized with a second polarity in an opposite circumferential direction. The magnetic field generated by the magnetized steel belt is detected by two magnetic detection elements, which are set so that the magnetic field detection directions of the two magnetic detection elements are parallel to the side surface of the tire, and the two elements are juxtaposed in a direction perpendicular to the side surface of the tire.