ON-BOARD DEVICE, CONTROL SYSTEM, CONTROL CIRCUIT, STORAGE MEDIUM, AND TRANSMISSION CONTROL METHOD

An on-board device in a control system in which frequencies used for transmitting and receiving a signal between the on-board device and an electronic key are known, and the electronic key transmits a response signal to a desired signal within a specified period when completing reception of the desired signal from the on-board device, the on-board device transmits the desired signal on a first frequency included in a specified frequency band; transmits a dummy signal on a second frequency included in the specified frequency band and different from the first frequency; receives the response signal to the desired signal from the electronic key; and controls to transmit the dummy signal in a period from before stopping transmitting the desired signal to after starting reception of the response signal.