A transmission apparatus having a plurality of network interfaces and a transmission method using a plurality of the network interfaces are provided. The transmission apparatus includes: a session information extraction unit which extracts session information of an IP packet; a network interface determination unit which determines a network interface associated with session information of the IP packet based on a table for mapping sessions to the network interfaces according to available bandwidths of the network interfaces; and a traffic distribution unit which transmits the IP packet to the determined network interface. Accordingly, traffic is distributed over a plurality of network interfaces, so that optimal communication performance can be ensured in a simultaneous multiple-mode connection environment, and network resources can be efficiently utilized.