A communication system includes a first device configured to transmit a signal to each of a first transmission path and a second transmission path provided within a vehicle, a control device configured to control an in-vehicle device on the basis of the signal transmitted to the second transmission path and transmit a signal indicating a result of control to a third transmission path, and a second device configured to detect a fraud state in the vehicle by performing a comparison on the basis of first information included in the signal received from the first transmission path and second information included in the signal received from the third transmission path.