A MAC device guarantees QoS in a VoIP wireless LAN by giving higher priority to voice data than to non-voice data. Transmission data generated by an application program is provided to a modem according to the priority assigned to the data. A kernel classifies the transmission data into voice and non-voice data. A MAC driver stores the voice and non-voice data in two queues, and transmits a MAC header to a MAC transmitter to notify it of generation of the transmission data, and transmits the stored voice or non-voice data to the MAC transmitter according to the type of a transmission interrupt received from the MAC transmitter in response to the notification. The MAC transmitter combines the voice or non-voice data, received from the MAC driver in response to the transmission interrupt, with a corresponding header, and outputs it to the modem.