Tire condition monitoring apparatus

When registering the ID code of a transmitter in a receiver, a commander is brought close to the transmitter and then a manipulation switch of the commander is pressed. Then, the commander transmits an instruction signal to the transmitter. In response to the instruction signal, the transmitter transmits a response signal including eight consecutive data frames. When receiving the signal from the transmitter, the receiver counts the number of the data fames in the received signal. f the number of the data frames is eight, the receiver determines that the received signal is a response signal generated based on manipulation of the commander and registered the ID code in the response signal. That is, the receiver determines whether to register the ID code in the received signal based on the number of the data frames included in the received signal. As a result, the ID code of the transmitter is easily registered in the receiver.