An on-board network system comprising: a main arithmetic unit configured to integrally control a plurality of on-board devices; and a plurality of control devices (61- 64, 81 - 83) connected to the main arithmetic unit via a communication network, each of the plurality of control devices (61- 64, 81 - 83) receiving information for controlling an on-board device to be controlled by the control device (61- 64, 81 - 83) from the main arithmetic unit via the communication network and controlling the on-board device based on the information received, wherein the communication network includes a ring-like communication path in a ring shape and is configured such that each of the control devices (61- 64, 81 - 83) is communicative with the main arithmetic unit and/or other one(s) of the control devices (61- 64, 81 - 83) via at least two communication paths (RN1 - RN8) of the ring-like network, in each of the communication paths (RN1 - RN8), a priority order of communications is determined according to a type of communication data, and the main arithmetic unit or one(s) of the control devices (61- 64, 81 - 83) is configured to adjust traffic of the communication data in other one(s) of the communication paths (RN1 - RN8) in accordance with the priority order of the other one(s) of the communication paths (RN1 - RN8) if a communication disturbance is detected in one of the communication paths (RN1 - RN8), wherein the plurality of control devices (61- 64, 81 - 83) include: a first control device (61 - 64) for controlling a body-related device and a second control device (61 - 64) on the ring-like communication path, and a third control device that is a powertrain ECU on a branch communication path that branches off from the ring-like communication path and merges into the ring-like communication path again, and if a communication disturbance is detected in the ring-like communication path, traffic of communication data among the first control device, the second control device, and the third control device (61- 64, 81) that is a powertrain ECU is adjusted based on the priority order in the branch communication path. a main arithmetic unit configured to integrally control a plurality of on-board devices; and a plurality of control devices (61- 64, 81 - 83) connected to the main arithmetic unit via a communication network, each of the plurality of control devices (61- 64, 81 - 83) receiving information for controlling an on-board device to be controlled by the control device (61- 64, 81 - 83) from the main arithmetic unit via the communication network and controlling the on-board device based on the information received, wherein the communication network includes a ring-like communication path in a ring shape and is configured such that each of the control devices (61- 64, 81 - 83) is communicative with the main arithmetic unit and/or other one(s) of the control devices (61- 64, 81 - 83) via at least two communication paths (RN1 - RN8) of the ring-like network, in each of the communication paths (RN1 - RN8), a priority order of communications is determined according to a type of communication data, and the main arithmetic unit or one(s) of the control devices (61- 64, 81 - 83) is configured to adjust traffic of the communication data in other one(s) of the communication paths (RN1 - RN8) in accordance with the priority order of the other one(s) of the communication paths (RN1 - RN8) if a communication disturbance is detected in one of the communication paths (RN1 - RN8), wherein the plurality of control devices (61- 64, 81 - 83) include: a first control device (61 - 64) for controlling a body-related device and a second control device (61 - 64) on the ring-like communication path, and a third control device that is a powertrain ECU on a branch communication path that branches off from the ring-like communication path and merges into the ring-like communication path again, and if a communication disturbance is detected in the ring-like communication path, traffic of communication data among the first control device, the second control device, and the third control device (61- 64, 81) that is a powertrain ECU is adjusted based on the priority order in the branch communication path.