In an on-board communication system, an on-board device transmits detection signals from a plurality of antennas disposed in a vehicle, and a plurality of portable devices each receive the detection signals and transmit response signals corresponding to the received detection signals. Each portable device measures an elapsed time from when the processing start signal is received, and (i) transmits a first response signal that includes information corresponding to the received detection signals, and (ii) when a specified time has elapsed from receipt of the processing start signal, transmits a second response signal including the same information as the information included in the first response signal. Position detection of the portable devices is performed based on the response signals.