A fault detection device capable of detecting network faults by itself with high accuracy in multi-vendor environments, without the need to interoperate with an associated device according to an identical protocol. A monitoring control packet transmitter generates a fault monitoring control packet and transmits the generated packet to the associated device with which the fault detection device need not interoperate to detect faults according to the same protocol. A transmit packet counter keeps count of the transmitted fault monitoring control packet. A receive packet counter receives a control packet transmitted from the associated device, and keeps count of the received control packet. A fault detector monitors the count of transmitted packets and the count of received packets and, if at least one of the counts remains unchanged for a fixed period of time, judges that a fault has occurred and sends a fault notification to outside.