In a communication network system connected between a transmission node and a reception node, both the transmission node and the reception node store the same secret information with their secret information storage units. The transmission node includes a counter configured to increase its transmission count value by 1 for transmitting each message, wherein a MAC generator generates MAC based on secret information, transmission data, and its transmission count value, thus transmitting a message including transmission data and MAC. The reception node includes a counter configured to increase a reception count value by 1 for receiving each message, wherein a MAC checking part generates MAC based on secret information, reception data, and its reception count value, obtained from the received message, thus checking whether the generated MAC matches the MAC obtained from the received message.