A medium access control method in a wireless LAN system using multiple channels includes: determining the size of data frames related to a plurality of wireless stations connected through multiple channels; adjusting the number or length of the data frames related to the respective wireless stations, based on the result of determination, so that transmission start time and transmission end time of the data frames coincide during a transmission duration of an access point; and transmitting the data frames, the number or length of which has been adjusted, to the respective wireless stations.