A fault that cannot be autonomously detected by all or a specific software module and all or a specific hardware device which realize the packet transmitting/receiving function is detected, and also a fault part is specified. A packet relay device realizes the packet transmitting and receiving functions through plural software modules and plural hardware devices. The packet relay device includes a memory space 105, reception statistics 131, 133, 135, and transmission statistics 132, 134, 136, and collects pass information of fault monitoring packets in all or the specific software module and all of the hardware devices whose operation is to be confirmed by the fault monitoring packets for detecting the faults within the device. A fault part specifying unit 118 is provided for specifying the fault part according to the pass information of the fault monitoring packet when abnormality is detected.