An on-vehicle gateway device connected to an information system network and a control system network of a vehicle executes monitoring the status of an information system via an information system access circuit taking charge of message transmission and reception to and from the information system network, and an information system management step to manage information acquired by the information system monitoring, monitoring the status of a control system via a control system access circuit taking charge of message transmission and reception to and from the control system network, and a control system management step to manage information acquired by the control system monitoring, managing policies for access control by the access control circuit controlling data flows between the information system access circuit and the control system access circuit, and determining whether or not to update the policies managed by policy management and to update the policies.