A power-saving network unit, which is connected to a network made up of a plurality of power-saving network units, includes: network monitoring means; network information memory; power-saving mode setting means; peripheral I/O interface; and digital processor. The network monitoring means monitors a topology of the network, or the interconnection relationship among the power-saving network units. Every time the network has been modified, the network monitoring means stores the modified network topology on the network information memory. The power-saving mode setting means receives the network information stored on the network information memory. If the power-saving network unit is a master or relay node in the network, then the power-saving mode setting means locks the peripheral I/O interface and digital processor of the power-saving network unit to the normal operation mode and prohibits these sections from entering the power-saving mode.