A communication system includes a user terminal connected to a first communication line, a device connected to the first communication line, and storing data to be updated, and a storage apparatus connected to a second communication line and storing the data transmitted from the device. A communication originated at the storage apparatus and addressed to the device and the user terminal is blocked by a firewall apparatus while a communication from the storage apparatus to the user terminal in reply to a request from the user terminal is permitted by the firewall apparatus.