PROBLEM TO BE SOLVED: To achieve link aggregation without adding a function to a reception side device.SOLUTION: In a femto gateway 30, which is a relay device, deletion processing units 34, 35, header conversion processing units 38, 39, and a translation processing unit 42 as a "management unit" manage correspondence relation between an intra-flow number in a TCP flow and an intra-link number given by TCP processing units 36, 37 regarding TCP-O packets distributed to a plurality of communication links by a distribution unit 33. Then, the "management unit" translates a first reception acknowledgement transmitted from the reception side device on the basis of the intra-flow number of each TCP-O packet transferred via a distribution destination link into a second reception acknowledgement based on the intra-link number on the basis of the correspondence relation, and distributes it to the TCP processing units 36, 37.