A diagnostic method performed by a gateway in a vehicle network, wherein the gateway comprises a controller and a physical (PHY) layer which manages ports and is connected to the controller, may comprise receiving, by the controller, a diagnostic request message from an external diagnostic apparatus connected to a first port; receiving, by the controller, a security authentication request message from the external diagnostic apparatus when a security authentication is required for the diagnostic request message; verifying, by the controller, the security authentication request message; and activating, by the PHY layer, a port of the PHY layer connected to a target communication node to be diagnosed among the ports, under control of the controller, when verification of the security authentication request message is completed.