Devices between which packets are transmitted and received include mutually corresponding packet counters. The same random number value is given to the packet counters as their initial values and the packet counters are updated with packet transmission/reception. The transmission-side device generates a MAC value, draws out part thereof on the basis of a counted value of its own packet counter, sets it as a divided MAC value, generates a packet by adding the value to a message and transmits the packet onto a network. The reception-side device generates a MAC value on the basis of the message in the received packet, draws out part thereof on the basis of a counted value of its own packet counter, compares the part with the divided MAC value in the received packet and thereby performs message authentication.