A device for controlling the fusion of a heterogeneous system, which receives a first data stream from a MAC layer of a WLAN through a LLC sub-layer and a second data stream from a MAC layer of a cellular network through a PDCP and a RLC sub-layer, and for fusing the first data stream and the second data stream into one data stream and transmitting the fused one data stream to a network layer. The device demultiplexes the data stream from the network layer into a third data stream corresponding to the MAC layer of the WLAN and a fourth data stream corresponding to the MAC layer of the cellular network, and transmits the third data stream to the MAC layer of the WLAN through the LLC and the fourth data stream to the MAC layer of the cellular network through the PDCP and the RLC sub-layer.