This paper presents an overview of the device which semi permanently supervises low tire pressure of driving or parking car, including low spare tire pressure. Such continual supervision is attained by the application of wireless system. The sensor on the tire has no power supply. It transmits echo radio wave by the excitement of its element, which is originated by the transporting radio signal from the detector on the car body. The device employs fail-safe system and its key components are consisted of passive elements, so that the device provides high reliability.