According to an embodiment, a communication system includes a transmitting apparatus and a receiving apparatus. The transmitting apparatus generates a pseudorandom number based on an algorithm, synchronously with the receiving apparatus; generates a communication frame identifier including a frame identifier indicating a type of a communication frame and the pseudorandom number to generate the communication frame; and transmits the communication frame. The receiving apparatus generates a pseudorandom number based on the algorithm, synchronously with the transmitting apparatus; receives the communication frame; compares the pseudorandom number included in the communication frame identifier with the pseudorandom number generated by the receiving apparatus and compares the frame identifier included in the communication frame identifier with a predetermined frame identifier; and performs one of making use of the communication frame, discarding the communication frame, and determining that the communication frame is an attack from an unauthorized party, based on comparison results.