A communication system for a vehicle includes: a plurality of controllers mounted on the vehicle; a data communication network for coupling the plurality of controllers with each other so that the controllers transmit data to and receive data from each other via the data communication network; a failure diagnosis device for diagnosing whether each controller is normal; a failure diagnosis activation device for activating the failure diagnosis device; and an avoiding condition setting device for setting an avoiding condition, which provides to avoid execution of the failure diagnosis when the failure diagnosis device is activated.