Electromagnetic field transmissions between a sensor unit mounted on the inside surface of a tire and a sensor control unit mounted on the vehicle body are attenuated by the wall surface of a tire disposed between the units, and sensitivity is reduced. A sealant is located on one of the motherboards so as to surround the peripheral edge portion of the display forming portion, and end spacers for maintaining a gap between the two motherboards and a tacker covering the end spacers are located on the end portions of the motherboard. An output power connector on the rotatable member can couple to an electrical cord at least partially wound about the rotatable member.