Vehicle-mounted communication system, relay device, and relay method

A vehicle-mounted communication system includes: a plurality of vehicle-mounted devices; and a relay device, wherein a first vehicle-mounted device of the vehicle-mounted devices transmits a frame addressed to a second vehicle-mounted device of the vehicle-mounted devices to the relay device, the frame being a frame for transmission and reception of information between the first vehicle-mounted device and the second vehicle-mounted device, and the relay device determines whether or not relay of the frame to the second vehicle-mounted device is permissible based on a result of comparison between a required safety level and a system safety level, the required safety level being a level of safety required for communication between the first vehicle-mounted device and the second vehicle-mounted device, the system safety level being at least based on function safety levels of the first vehicle-mounted device and the second vehicle-mounted device regarding safety of a vehicle.