1. A relay transmission device for transmitting data from a transmission terminal to a reception terminal via a plurality of relay devices, said relay transmission device comprising: transmission buffers each of which corresponds to, among the plurality of relay devices, a different one of relay devices that are data transmission destinations, and temporarily holds data to be transmitted from said relay transmission device to the corresponding relay device; a cooperative party determination unit operable to determine one or more cooperative relay devices that are to cooperate with said relay transmission device, from among the plurality of relay devices; a transmission flow information management unit operable to, for each cooperative relay device, exchange information including a data remaining amount of a transmission buffer with the cooperative relay device, the data remaining amount being an amount of data held in the transmission buffer; a self-optimization flow control unit operable to, for each cooperative relay device, determine an amount of data to be transmitted from said relay transmission device to the cooperative relay device so that respective data remaining amounts of said transmission buffers are equalized; a total optimization flow control unit operable to, for each cooperative relay device, determine an amount of data to be transmitted from said relay transmission device to the cooperative relay device so that a data remaining amount of said transmission buffers and a data remaining amount of transmission buffers included in the cooperative relay device are equalized; an optimization coefficient storage unit in which a self-optimization coefficient and a total optimization coefficient are stored for each cooperative relay device, the self-optimization coefficient being a coefficient for weighting the amount determined by said self-optimization flow control unit, and the total optimization coefficient being a coefficient for weighting the amount determined by said total optimization flow control unit; and a transmission buffer management unit operable to, for each cooperative relay device, calculate a sum of a value obtained by weighting the amount determined by said self-optimization flow control unit based on the self-optimization coefficient and a value obtained by weighting the amount determined by said total optimization flow control unit based on the total optimization coefficient, and transmit data of an amount specified by the calculated sum to the cooperative relay device.