In a packet transmission device and control method thereof which can improve the transmission rate of a packet without adding a high speed user data-conversion card, when packet data is inputted through a single arbitrary channel among a plurality of channels, from among a predetermined number of low speed user data-conversion cards provided common to the plurality of channels, the low speed user data-conversion cards by a number corresponding to a ratio between an output transmission rate of the low speed user data-conversion card and an input transmission rate of the packet data are extracted. The packet data is sequentially and cyclically distributed to the low speed user data-conversion cards extracted. Predetermined length user data outputted from the low speed user data-conversion cards are converted into a frame of HSDPA (High Speed Downlink Packet Access) system and multiplexed into a single channel HS-DSCH (High Speed Downlink Shared Channel) to be outputted.