A network device includes: a connector connected to a network; a receiving line connected to the connector; a transmitting line connected to the connector; a communication control unit configured to control communication with an external network device by receiving a signal from the network through the connector and the receiving line, and transmitting the signal to the network through the transmitting line and the connector; and a loop-back processing unit configured to loop back the signal transmitted through the receiving line, to the transmitting line when the communication control unit is in a power-off state or a standby state.