A communication control system comprises: a router that connects a first network and a second network; a communication relay device that connects the first network and the second network; and a hardware processor that: determines whether a first packet received from the first network is a packet to be discarded by the router; transmits a second packet corresponding to the first packet to the second network when the first packet is determined as the packet to be discarded by the router; and rewrites a transmitter IP address of a response packet corresponding to the second packet to an IP address of the router and sends the response packet to the first network when the response packet is received from the second network.