A vehicle interior illumination system comprising: a master control unit; a plurality of slave control units configured to perform multiplex communication with the master control unit and control a light source mounted on a vehicle in accordance with an instruction of the master control unit, wherein the slave control units comprise a storage unit configured to store setting information; a multiplex communication line configured to connect the master control unit to the plurality of slave control units; and characterized by a wireless control unit that is connected to the multiplex communication line and configured to perform wireless communication with an external device, wherein the wireless control unit is configured to receive setting information on vehicle interior illumination from the external device and transmit the setting information received from the external device to the master control unit via the multiplex communication line, wherein in response to reception of the setting information from the wireless control unit, the master control unit transmits, to the slave control unit, an instruction to rewrite the setting information stored in the storage unit to the setting information received from the wireless control unit, wherein the master control unit is configured to transmit a control instruction to the slave control unit to control the light source in accordance with the setting information received from the wireless control unit, and wherein the slave control unit controls the light source in accordance with the setting information stored in the storage unit in response to reception of the control instruction from the master control unit. a master control unit; a plurality of slave control units configured to perform multiplex communication with the master control unit and control a light source mounted on a vehicle in accordance with an instruction of the master control unit, wherein the slave control units comprise a storage unit configured to store setting information; a multiplex communication line configured to connect the master control unit to the plurality of slave control units; and characterized by a wireless control unit that is connected to the multiplex communication line and configured to perform wireless communication with an external device, wherein the wireless control unit is configured to receive setting information on vehicle interior illumination from the external device and transmit the setting information received from the external device to the master control unit via the multiplex communication line, wherein in response to reception of the setting information from the wireless control unit, the master control unit transmits, to the slave control unit, an instruction to rewrite the setting information stored in the storage unit to the setting information received from the wireless control unit, wherein the master control unit is configured to transmit a control instruction to the slave control unit to control the light source in accordance with the setting information received from the wireless control unit, and wherein the slave control unit controls the light source in accordance with the setting information stored in the storage unit in response to reception of the control instruction from the master control unit.