A packet relay apparatus, which is configured to transmit from a mirror port a mirror packet copied from one of a packet to be received and a packet to be transmitted, the packet relay apparatus comprising: a packet receiving module configured to receive a packet from an input port; a security judgment module configured to judge whether or not the packet is possibly one of an attack and an attack sign; a mirror processing module configured to generate, when it is judged that the packet is possibly one of an attack and an attack sign, a replica of the packet as the mirror packet; and a transmitting module configured to transmit the mirror packet from the mirror port.