A repeater is used in an in-vehicle communication system for retransmitting communication frames between communication nodes. The repeater includes a storage unit, a retransmission unit, a determiner, and a generator. The storage unit stores a routing map that associates a frame ID that identifies a communication frame from a communication node and a destination of the communication frame. The retransmission unit retransmits the communication frame between the communication nodes based on the routing map stored in the storage unit. The determiner determines whether a preset condition for generating the routing map is satisfied. The generator generates the routing map based on information in the communication frame received from the communication node and stores the map in the storage unit when the determiner determines that the preset condition is satisfied.