Abstract: A device for monitoring the air pressure of a wheel. The device prevents a decrease in the transmission level of radio waves caused by impedance mismatch between an antenna, which radiates the radio waves, and a circuit, which produces signals that are to be radiated as the radio waves. The device includes a valve stem through which air is charged. The valve stem extends through a vehicle wheel. A transmitter is secured to the wheel to transmit a signal representing the air pressure of the wheel to a receiver installed in the vehicle. The device further includes a case attached to the wheel. The case is connected to the valve stem. An electric circuit is accommodated in the case to detect the air pressure and convert the detected pressure to an electric signal. An antenna radiates the signal produced by the electric circuit and is arranged about the valve stem. A conveying mechanism conveys the signals produced by the electric circuit to the antenna.