To suppress the rise of a load on a bus (vehicle-mounted electronic control network) by allowing a gateway ECU to exclude such an erroneous message as to be revoked even if it is received by an ECU node.

A vehicle-mounted gateway device 1 is equipped with a message normality confirming means 61 to check and confirm the normality of contents of the message received from a first vehicle-mounted electronic control network CAN1 or a second vehicle-mounted electronic control network CAN2 and a message forwarding control means 62 to make control so that a message determined to be abnormal normal by the message normality confirming means 61 is not forwarded to the second or the first vehicle-mounted electronic control network CAN2/CAN1.