A vehicle on-board communication device comprising: vehicle on-board instruments (12, 14) that are mounted in a vehicle and are connected to a communication line, and that have a storage that stores data; an acquisition unit that acquires divided updating data that is created by dividing updating data which updates the data stored in the storage of the vehicle on-board instruments (12, 14); a relay unit that delivers the divided updating data acquired by the acquisition unit to the vehicle on-board instruments (12, 14) via the communication line; an instrument load transmission interval deciding unit that determines a processing load based on a processing state of at least one of the vehicle on-board instruments (12, 14), the acquisition unit, and the relay unit, and selects a transmission interval for the divided updating data based on this processing load; and a delivery control unit that controls the relay unit such that the divided updating data is delivered to the vehicle on-board instruments (12, 14) at transmission intervals which are not less than the transmission interval selected by the instrument load transmission interval deciding unit; the vehicle on-board communication device further comprising a communication line load transmission interval deciding unit that determines a load of the communication line based on a communication state of the communication line, and selects a transmission interval for the divided updating data based on this load, wherein the delivery control unit compares the transmission interval selected by the communication line load transmission interval deciding unit with the transmission interval selected by the instrument load transmission interval deciding unit, and controls the relay unit such that the divided updating data is delivered to the vehicle on-board instruments (12, 14) at a transmission interval that is equal to or greater than the longer of these transmission intervals. vehicle on-board instruments (12, 14) that are mounted in a vehicle and are connected to a communication line, and that have a storage that stores data; an acquisition unit that acquires divided updating data that is created by dividing updating data which updates the data stored in the storage of the vehicle on-board instruments (12, 14); a relay unit that delivers the divided updating data acquired by the acquisition unit to the vehicle on-board instruments (12, 14) via the communication line; an instrument load transmission interval deciding unit that determines a processing load based on a processing state of at least one of the vehicle on-board instruments (12, 14), the acquisition unit, and the relay unit, and selects a transmission interval for the divided updating data based on this processing load; and a delivery control unit that controls the relay unit such that the divided updating data is delivered to the vehicle on-board instruments (12, 14) at transmission intervals which are not less than the transmission interval selected by the instrument load transmission interval deciding unit; the vehicle on-board communication device further comprising a communication line load transmission interval deciding unit that determines a load of the communication line based on a communication state of the communication line, and selects a transmission interval for the divided updating data based on this load, wherein the delivery control unit compares the transmission interval selected by the communication line load transmission interval deciding unit with the transmission interval selected by the instrument load transmission interval deciding unit, and controls the relay unit such that the divided updating data is delivered to the vehicle on-board instruments (12, 14) at a transmission interval that is equal to or greater than the longer of these transmission intervals.