The present invention relates to a wireless communication system, and more specifically, to a method and apparatus for accessing channels in a wireless LAN system. The method for accessing channels in a station (STA) of the wireless communication system according to one embodiment of the present invention comprises the steps of transmitting a first frame of a channel access request to an access point (AP); and receiving, from the AP, a second frame related to whether access to the channel is allowed in response to the first frame, wherein the second frame can include a value for a channel access start offset with respect to the STA.