IN-VEHICLE NETWORK SYSTEM

PROBLEM TO BE SOLVED: To diagnose an ECU while a setting state of a certain service is altered even if a network configuration is complicated.SOLUTION: A gateway device 1a counts an elapsed time since the receipt of a session transition request from a diagnosis device 2 to the ECU for requesting a transition from an initial session to a diagnosis session using a session timer A, estimates a session state of the ECU recognized by the diagnosis device 2 based on the elapsed time, and transmits a session state holding request to the ECU to instruct the ECU to hold the state of the diagnosis session so as not to be transited to the initial session from the diagnosis session when the session state recognized by the diagnosis device 2 is the diagnosis session.