A portable device performs two-way communication with an on-board device. The portable device has an operating circuit, a communication circuit, and a controller circuit. The operating circuit accepts an operation for stopping communication with the on-board device. The communication circuit receives a first signal from the on-board device and transmits to the on-board device a second signal including information on reception strength of the first signal and information on communication stop corresponding to the operation accepted by the operating circuit. When the reception strength of the first signal received by the communication circuit is less than or equal to a first threshold, the controller circuit causes the communication circuit to stop the communication with the on-board device.