A system includes a plurality of in-vehicle devices and having a function of determining abnormality of communication in the devices by authenticating a message including data, a first counter counted each time a power source is turned on, and a second counter incremented over time. The system includes an authentication unit for performing, when a message including control data to be exchanged between the devices is authenticated, authentication on a message sent and received between the devices and including at least the control data and the first counter by using at least the control data and the first counter, in which when it is confirmed whether synchronization is achieved between the devices, the authentication unit is for performing authentication on a message sent and received between the devices and not including the first counter but including the second counter by using the second counter without using the first counter.