In a mobile communication system according to the present invention, an AP/ULP layer of a transmission-side node (A) is configured to acquire, from a SCTP layer of the transmission-side node (A), a “rwnd” for each connection of the SCTP layer, a “cwnd” for each physical path, and a third data amount indicating an amount of data transmitted within a unit time by the transmission-side node (A) for each stream; and when an urgency flag is included in data subject to transmission, the AP/ULP layer of the transmission-side node (A) is configured to determine a stream and a physical path through which the data subject to transmission should be transmitted, based on the “rwnd”, the “cwnd”, and the third data amount, and to notify, to the SCTP layer, the determination.