A communication system, comprising: a mobile device including: a signal generating unit configured to generate a first signal, the first signal including frequency information that is used in wireless communication between the mobile device and a tuner of an in-vehicle electronic device, a low frequency transponder response unit configured to modulate the first signal generated by the signal generating unit into LF-band radio waves and to wirelessly transmit the radio waves; and a radio frequency transmitting unit configured to modulate a response signal into RF-band radio waves and to wirelessly transmit the radio waves; the tuner including: a setting processing unit configured to set a receiving frequency of an antenna and a demodulation frequency of a phase synchronization circuit, the setting processing unit being configured to set the receiving frequency and the demodulation frequency on the basis of the frequency information included in the first signal from the mobile device; the in-vehicle electronic device further comprising: a setting information acquisition unit configured to acquire setting information corresponding to the frequency information, the frequency information being included in the first signal received by an engine switch, wherein the setting processing unit is configured to set the receiving frequency of the antenna on the basis of the setting information acquired by the setting information acquisition unit; the engine switch is configured to carry out bidirectional communication with LF-band radio waves; and the LF transponder response unit is configured to carry out bidirectional communication with the engine switch when the mobile device is cataloged in the in-vehicle electronic device, and the first signal is transmitted from the mobile device to the in-vehicle electronic device when the mobile device is cataloged in the in-vehicle electronic device. a mobile device including: a signal generating unit configured to generate a first signal, the first signal including frequency information that is used in wireless communication between the mobile device and a tuner of an in-vehicle electronic device, a low frequency transponder response unit configured to modulate the first signal generated by the signal generating unit into LF-band radio waves and to wirelessly transmit the radio waves; and a radio frequency transmitting unit configured to modulate a response signal into RF-band radio waves and to wirelessly transmit the radio waves; a signal generating unit configured to generate a first signal, the first signal including frequency information that is used in wireless communication between the mobile device and a tuner of an in-vehicle electronic device, a low frequency transponder response unit configured to modulate the first signal generated by the signal generating unit into LF-band radio waves and to wirelessly transmit the radio waves; and a radio frequency transmitting unit configured to modulate a response signal into RF-band radio waves and to wirelessly transmit the radio waves; the tuner including: a setting processing unit configured to set a receiving frequency of an antenna and a demodulation frequency of a phase synchronization circuit, the setting processing unit being configured to set the receiving frequency and the demodulation frequency on the basis of the frequency information included in the first signal from the mobile device; the in-vehicle electronic device further comprising: a setting information acquisition unit configured to acquire setting information corresponding to the frequency information, the frequency information being included in the first signal received by an engine switch, wherein the setting processing unit is configured to set the receiving frequency of the antenna on the basis of the setting information acquired by the setting information acquisition unit; the engine switch is configured to carry out bidirectional communication with LF-band radio waves; and the LF transponder response unit is configured to carry out bidirectional communication with the engine switch when the mobile device is cataloged in the in-vehicle electronic device, and the first signal is transmitted from the mobile device to the in-vehicle electronic device when the mobile device is cataloged in the in-vehicle electronic device. a setting information acquisition unit configured to acquire setting information corresponding to the frequency information, the frequency information being included in the first signal received by an engine switch, wherein the setting processing unit is configured to set the receiving frequency of the antenna on the basis of the setting information acquired by the setting information acquisition unit; the engine switch is configured to carry out bidirectional communication with LF-band radio waves; and the LF transponder response unit is configured to carry out bidirectional communication with the engine switch when the mobile device is cataloged in the in-vehicle electronic device, and the first signal is transmitted from the mobile device to the in-vehicle electronic device when the mobile device is cataloged in the in-vehicle electronic device.