A communication system has: a portable communication device; and an in-vehicle communication device that communicates with the portable communication device through synchronous communication. The potable communication device transmits dummy signals simulating the vehicle information to the in-vehicle communication device in a first period. The in-vehicle communication device learns and records a first filter coefficient for filtering the dummy signals that are received in the first period and a second filter coefficient for filtering signals that are received from the sensors in a second period in which the portable communication device do not transmit the dummy signals, and the in-vehicle communication device calculates a third filter coefficient for removing noises on signals received from the sensors based on the first and second filter coefficients and then filters the received signals using the third filter coefficient.