A communication system comprising: a first network connected to lower-layer equipment (31, 32, 33); a second network connected to a higher-layer application (60, 61, 62); a gateway device connected to the first network and the second network; and a tunnel device (100, 300) connected to the first network and the second network, the tunnel device bypassing the gateway device; wherein the communication system is configured so that, if the application transmits a network connection request to the tunnel device (100, 300), then a network connection response is returned to the application (60, 61, 62); and if the application (60, 61, 62) receives the network connection response, then the application (60, 61, 62) sets up a virtual tunnel for connecting the application (60, 61, 62) to the tunnel device (100, 300), and directly communicates with the equipment (31, 32, 33) via the virtual tunnel; the tunnel device (100, 300) includes: a priority control processing unit configured to perform a priority control process based on priority information for data to be communicated between the first network and the second network via the tunnel device (100, 300) to give high priority to processing sensor data and control data and to give low priority to processing setting data and diagnosis information; and a tunnel device synchronization unit configured to synchronize and equalize mutual setting information with another tunnel device for backup connected to the first network and the second network to improve a fault tolerance of the tunnel device (100, 300). a first network connected to lower-layer equipment (31, 32, 33); a second network connected to a higher-layer application (60, 61, 62); a gateway device connected to the first network and the second network; and a tunnel device (100, 300) connected to the first network and the second network, the tunnel device bypassing the gateway device; wherein the communication system is configured so that, if the application transmits a network connection request to the tunnel device (100, 300), then a network connection response is returned to the application (60, 61, 62); and if the application (60, 61, 62) receives the network connection response, then the application (60, 61, 62) sets up a virtual tunnel for connecting the application (60, 61, 62) to the tunnel device (100, 300), and directly communicates with the equipment (31, 32, 33) via the virtual tunnel; the tunnel device (100, 300) includes: a priority control processing unit configured to perform a priority control process based on priority information for data to be communicated between the first network and the second network via the tunnel device (100, 300) to give high priority to processing sensor data and control data and to give low priority to processing setting data and diagnosis information; and a tunnel device synchronization unit configured to synchronize and equalize mutual setting information with another tunnel device for backup connected to the first network and the second network to improve a fault tolerance of the tunnel device (100, 300). a priority control processing unit configured to perform a priority control process based on priority information for data to be communicated between the first network and the second network via the tunnel device (100, 300) to give high priority to processing sensor data and control data and to give low priority to processing setting data and diagnosis information; and a tunnel device synchronization unit configured to synchronize and equalize mutual setting information with another tunnel device for backup connected to the first network and the second network to improve a fault tolerance of the tunnel device (100, 300).