A communication system, comprising: a first device (1_2) and a second device (1_1) which are mutually coupled via a network so as to transmit and receive packets over the network, wherein the first device and the second device respectively include a first packet counter and a second packet counter, wherein the same random number value is given to the first (6_2) and second (6_1) packet counters as their initial values and the first and second packet counters are adapted to be respectively updated in association with transmission and reception of packets, wherein when a message is to be transmitted to the second device (1_1), the first device (1_2) is adapted to generate one message authentication code on the basis of the message, to draw out part of the message authentication code on the basis of a counted value of the first packet counter, to set the part as a divided message authentication code, to generate a packet which includes the message and the divided message authentication code and to transmit the packet to the second device over the network, and wherein, when the packet has been received from the first device, the second device (1_1) is adapted to generate another message authentication code on the basis of the message included in the received packet, to draw out part of the message authentication code on the basis of a counted value of the second packet counter, to compare the drawn-out part of the message authentication code on the basis of the counted value of the second packet counter with the divided message authentication code included in the received packet and to thereby perform message authentication. a first device (1_2) and a second device (1_1) which are mutually coupled via a network so as to transmit and receive packets over the network, wherein the first device and the second device respectively include a first packet counter and a second packet counter, wherein the same random number value is given to the first (6_2) and second (6_1) packet counters as their initial values and the first and second packet counters are adapted to be respectively updated in association with transmission and reception of packets, wherein when a message is to be transmitted to the second device (1_1), the first device (1_2) is adapted to generate one message authentication code on the basis of the message, to draw out part of the message authentication code on the basis of a counted value of the first packet counter, to set the part as a divided message authentication code, to generate a packet which includes the message and the divided message authentication code and to transmit the packet to the second device over the network, and wherein, when the packet has been received from the first device, the second device (1_1) is adapted to generate another message authentication code on the basis of the message included in the received packet, to draw out part of the message authentication code on the basis of a counted value of the second packet counter, to compare the drawn-out part of the message authentication code on the basis of the counted value of the second packet counter with the divided message authentication code included in the received packet and to thereby perform message authentication.