A communication security apparatus includes a communicator that receives a packet from a first device and transmits the received packet to a second device, a memory that retains address authentication information containing pairs of a physical address and a logical address of one or more devices, and a controller. After a learning period of receiving and transmitting packets, the controller determines whether a pair of a physical address and a logical address of the first device and the second device match any one of the pairs of the physical address and the logical address of the one or more devices in the packet, and discards the packet when the pair of the physical address and the logical address of the first device and the second device do not match any one of the pairs of the physical address and the logical address of the one or more devices.