A communication network system connecting a plurality of micro-computing units, each of which comprises a transmitter configured to transmit a message, a receiver configured to receive the message, a counter, a message authentication code generator, a message authentication code checking part, and a storage unit configured to store secret information shared by the plurality of micro-computing units, wherein according to the micro-computing unit serving as a transmission node, the counter is configured to hold a transmission count value to be increased upon transmitting each message, the message authentication code generator is configured to generate a first message authentication code based on transmission data included in the message, extracted upper bits corresponding to a predetermined number of extracted bits from a bit string of the transmission count value, and the secret information such that the first message authentication code includes a number of sections each configured of a predetermined number of bits, in which one section of the first message authentication code specified by a value of predetermined extracted lower bits, other than the extracted upper bits, within the bit string of the transmission count value is used as an extracted part of the first message authentication code, the number of sections depending on the total number of bits of a calculated message authentication code value, and the transmitter is configured to transmit the message including the transmission data and the extracted part of the first message authentication code, and wherein according to the micro-computing unit serving as a reception node, the receiver is configured to receive the message from the transmitter, the counter is configured to hold a reception count value to be increased upon receiving each message, and the message authentication code checking part is configured to generate a second message authentication code based on reception data, which is obtained from the message received by the receiver, extracted upper bits corresponding to a predetermined number of extracted bits from a bit string of the reception count value, and the secret information such that the second message authentication code includes a number of sections each configured of the predetermined number of bits, in which one section of the second message authentication code specified by a value of predetermined extracted lower bits, other than the extracted upper bits, within the bit string of the reception count value is used as a checking part of the second message authentication code, the number of sections depending on the total number of bits of a calculated message authentication code value, thus checking whether the extracted part of the first message authentication code matches the checking part of the second message authentication code for authentication of the message. wherein according to the micro-computing unit serving as a transmission node, the counter is configured to hold a transmission count value to be increased upon transmitting each message, the message authentication code generator is configured to generate a first message authentication code based on transmission data included in the message, extracted upper bits corresponding to a predetermined number of extracted bits from a bit string of the transmission count value, and the secret information such that the first message authentication code includes a number of sections each configured of a predetermined number of bits, in which one section of the first message authentication code specified by a value of predetermined extracted lower bits, other than the extracted upper bits, within the bit string of the transmission count value is used as an extracted part of the first message authentication code, the number of sections depending on the total number of bits of a calculated message authentication code value, and the transmitter is configured to transmit the message including the transmission data and the extracted part of the first message authentication code, and wherein according to the micro-computing unit serving as a reception node, the receiver is configured to receive the message from the transmitter, the counter is configured to hold a reception count value to be increased upon receiving each message, and the message authentication code checking part is configured to generate a second message authentication code based on reception data, which is obtained from the message received by the receiver, extracted upper bits corresponding to a predetermined number of extracted bits from a bit string of the reception count value, and the secret information such that the second message authentication code includes a number of sections each configured of the predetermined number of bits, in which one section of the second message authentication code specified by a value of predetermined extracted lower bits, other than the extracted upper bits, within the bit string of the reception count value is used as a checking part of the second message authentication code, the number of sections depending on the total number of bits of a calculated message authentication code value, thus checking whether the extracted part of the first message authentication code matches the checking part of the second message authentication code for authentication of the message.