In an in-vehicle network system (100), a parent control unit (10) and a child control unit (20) constitute a control unit. In the in-vehicle network system (100), a monitoring unit (70) acquires communication data being communicated between the parent control unit (10) and the child control unit (20) and flowing in an in-vehicle network (30) which connects the parent control unit (10) and the child control unit (20) to each other. The monitoring unit (70) diagnoses an abnormality in the parent control unit (10) based on the communication data acquired by the monitoring unit (70) and diagnostic data stored by a memory unit.