An overlay network system comprises an overlay network composed of a plurality of nodes and a plurality of sub-overlay networks each of which is composed of a subset of the plurality of nodes and which are hierarchized. The overlay network is managed as a 0th-level sub-overlay network at the highest hierarchical level. Each of the plurality of sub-overlay networks is allocated a sub-overlay network ID whose number of bits corresponds to the hierarchical level of the network. The high-order one or more bits in the sub-overlay network ID also indicate the sub-overlay network ID of a sub-overlay network at the high hierarchical level corresponding to the number of the one or more bits.