Abstract: Provided is an on-board device and a vehicle communication system that can expand the transmission range of signals that are transmitted from transmission antennas of the on-board device. An on-board device transmits signals to a portable device, from a plurality of first to fourth LF transmission antennas that are provided for a vehicle at positions that are separate from each other. The on-board device includes a transmission unit that transmits the signals from the first to fourth LF transmission antennas such that a transmission range in which the portable device can receive the signals is a range around the vehicle. The transmission unit substantially simultaneously transmits the signals from two or more transmission antennas among the first to fourth LF transmission antennas, to expand the transmission range.