A network system that is part of a main system includes: a first PCI express-network bridge with a first control unit and a first PCI express adapter terminating a first PCI express bus; and a second PCI express-network bridge connected to the first PCI express-network bridge through a network. The second PCI express-network bridge includes a second control unit and a second PCI express adapter terminating a second PCI express bus, wherein the first control unit detects a destination of a packet sent from the first PCI express adapter, searches a physical address of the destination from a packet encapsulating table, and encapsulates the packet in a frame so that the frame includes the physical address, and wherein the second control unit removes the encapsulation tagged to the packet, and transfers the packet to the destination through the second PCI express bus by referring to a PCI express configuration register.