A communication system includes multiple nodes connected with each other. Each of the multiple nodes generates a message authentication code using a count value of a counter. The multiple nodes include a transmission node and a reception node. The count value of the counter is includes a high-order count value and a low-order count value. In the transmission node, a normal message generation portion generates a normal message to include a transmission data, the low-order count value, and the message authentication code, and a synchronization message generation portion generates a synchronization message. In the reception node, a message verification portion verifies the received normal message, a resynchronization request portion transmits a resynchronization request of the counter to the transmission node, and a count value update portion updates the high-order count value stored in the reception count value storage portion when the synchronization message is received.