A communication system includes a first electronic control unit configured to determine whether a reception message received from a communication bus corresponds to a communication message determined in advance to be transmitted to the communication bus by the first electronic control unit, determine whether the communication message is transmitted from the first electronic control unit to the communication bus, and output a notification signal that is a signal for causing the communication message that a second electronic control unit acquires from the communication bus to be deleted from the second electronic control unit when the first electronic control unit determines that the reception message corresponds to the communication message and determines that the communication message is not transmitted from the first electronic control unit to the communication bus.