A portable device for preventing communication errors caused by disturbance such as noise. The portable device receives a first request signal from a communication controller and transmits a first response signal corresponding to the first request signal. A transmitter transmits the first response signal with a selected one of a plurality of frequencies. A transmission control unit transmits the first response signal from the transmitter with the one of the plurality of frequencies. The portable device further receives a second request signal from the communication control unit, which receives the first response signal. When the second request signal cannot be received, the portable device retransmits the first response signal from the transmitter with a frequency that differs from the one of the plurality of frequencies.