To obtain a method and an apparatus by which a driver can always grasp the state of a tire pressure by installing an air-driven member by which an object to be induced is changed over to be in a continuity state or a short-circuit state and an exciting body which generates an induced signal, and judging whether the tire pressure is a normal state or not and generating or displaying a warning signal.

The air-driven member 65 is installed in the opening part 54 of a shell 63. The shell 63 is coupled or integrally molded inside a tire in such a way that its inside shows a sealed-up state. A tire pressure enters from the opening 64, and it pressed and moves the air-driven member 65. The movement end part of the air-driven member 65 changes over respective bodies 61, 62, to be induced, to a continuity state or a short-circuit state so as to correspond to the tire pressure. When the objects 61, 62 to be induced are in the short-circuit state, they are brought close to an exciting body within a specific distance due to the rotation of a tire, and the exciting body senses an induced magnetic-field signal so as to generate an induced voltage. On the basis of the number of times of occurrences of the detected induced voltage, whether the tire pressure is a normal pressure state or not is judged. When the tire pressure is not in the normal pressure state, a warning signal is issued or displayed.