An in-vehicle network system (1, 2), comprising: a power source; an upper electronic control unit; a first intermediate electronic control unit and a second intermediate electronic control unit, each of the first intermediate electronic control unit and the second intermediate electronic control unit being configured to receive electric power supply from the power source and perform communication with the upper electronic control unit; at least one first lower electronic control unit that is connected to the first intermediate electronic control unit via a first power source path and a first communication path, electric power supply to the first lower electronic control unit and communication management of the first lower electronic control unit being performed by the first intermediate electronic control unit; at least one second lower electronic control unit that is connected to the second intermediate electronic control unit via a second power source path and a second communication path, electric power supply to the second lower electronic control unit and communication management of the second lower electronic control unit being performed by the second intermediate electronic control unit; a power source relay provided between the first power source path and the second power source path; and a communication relay provided between the first communication path and the second communication path, wherein when the upper electronic control unit detects that an anomaly occurs in the first intermediate electronic control unit, the power source relay and the communication relay are controlled such that the first power source path and the second power source path are connected to each other and the first communication path and the second communication path are connected to each other, and the electric power supply to the first lower electronic control unit and the communication management of the first lower electronic control unit are performed by the second intermediate electronic control unit. a power source; an upper electronic control unit; a first intermediate electronic control unit and a second intermediate electronic control unit, each of the first intermediate electronic control unit and the second intermediate electronic control unit being configured to receive electric power supply from the power source and perform communication with the upper electronic control unit; at least one first lower electronic control unit that is connected to the first intermediate electronic control unit via a first power source path and a first communication path, electric power supply to the first lower electronic control unit and communication management of the first lower electronic control unit being performed by the first intermediate electronic control unit; at least one second lower electronic control unit that is connected to the second intermediate electronic control unit via a second power source path and a second communication path, electric power supply to the second lower electronic control unit and communication management of the second lower electronic control unit being performed by the second intermediate electronic control unit; a power source relay provided between the first power source path and the second power source path; and a communication relay provided between the first communication path and the second communication path, wherein when the upper electronic control unit detects that an anomaly occurs in the first intermediate electronic control unit, the power source relay and the communication relay are controlled such that the first power source path and the second power source path are connected to each other and the first communication path and the second communication path are connected to each other, and the electric power supply to the first lower electronic control unit and the communication management of the first lower electronic control unit are performed by the second intermediate electronic control unit.