An in-vehicle communication system comprising: a master control unit mounted on a vehicle; a plurality of slave devices mounted on the vehicle, wherein the master control unit is communicably connected to each of the slave devices: a plurality of buckles provided in association with each of a plurality of seats mounted on the vehicle; and at least one switch unit configured to generate a signal in accordance with an attachment and detachment state of at least one of the plurality of buckles, wherein the master control unit controls the plurality of slave devices based on the signal generated by the at least one switch unit; wherein the master control unit allocates a unique identification information to each of the plurality of slave devices in response to generation of the signal generated by the at least one switch unit; the system being characterized in that the master control unit recognizes a generation pattern of the signal when allocating the unique identification information to at least one of the plurality of slave devices in response to the generation of the signal generated by the at least one switch unit, and specifies the slave device to be allocated with the unique identification information. a master control unit mounted on a vehicle; a plurality of slave devices mounted on the vehicle, wherein the master control unit is communicably connected to each of the slave devices: a plurality of buckles provided in association with each of a plurality of seats mounted on the vehicle; and at least one switch unit configured to generate a signal in accordance with an attachment and detachment state of at least one of the plurality of buckles, wherein the master control unit controls the plurality of slave devices based on the signal generated by the at least one switch unit; wherein the master control unit allocates a unique identification information to each of the plurality of slave devices in response to generation of the signal generated by the at least one switch unit; the system being characterized in that the master control unit recognizes a generation pattern of the signal when allocating the unique identification information to at least one of the plurality of slave devices in response to the generation of the signal generated by the at least one switch unit, and specifies the slave device to be allocated with the unique identification information. a plurality of buckles provided in association with each of a plurality of seats mounted on the vehicle; and at least one switch unit configured to generate a signal in accordance with an attachment and detachment state of at least one of the plurality of buckles, wherein the master control unit controls the plurality of slave devices based on the signal generated by the at least one switch unit; wherein the master control unit allocates a unique identification information to each of the plurality of slave devices in response to generation of the signal generated by the at least one switch unit; the system being characterized in that the master control unit recognizes a generation pattern of the signal when allocating the unique identification information to at least one of the plurality of slave devices in response to the generation of the signal generated by the at least one switch unit, and specifies the slave device to be allocated with the unique identification information.