A transmitter is mounted on a wheel of a vehicle by a jig of an automatic mounting machine. The transmitter has a measuring-transmitting portion, a valve stem, and a casing. The measuring-transmitting portion measures data representing the conditions of the tire and transmits data. The valve stem is inserted into the wheel and allows air to flow into the tire. The casing accommodates the measuring-transmitting portion, and has a recessed portion. The jig of the automatic mounting machine is engaged with the recessed portion when the automatic mounting machine holds the valve stem. Therefore, even a valve stem with the casing is easily attached to a valve hole of the wheel by using the automatic mounting machine.