A network management apparatus interconnects a plurality of computers through physical lines having a predetermined line speed to manage a switch apparatus constructing a parallel computer system. The switch apparatus comprises a plurality of physical port units to enable or disable the same through individual power supply ON/OFF control for the physical port units, thereby changing and controlling the line transmission speed according to the enable number. A logical port constructing unit in the network management apparatus bundles the physical lines by a plurality of physical port units in the switch apparatus to construct logical lines. A port control unit changes the number of operations of a plurality of physical port units assigned to the logical port units through the power supply ON/OFF control according to a necessary data transmission speed for the logical port units, thereby dynamically changing the line transmission speed.