Method for operating sounding in wireless lan system, and apparatus therefor

An embodiment of the present invention suggests a method in which an access point (AP) obtains channel quality information in a wireless LAN system. The AP transmits, to at least one station (STA), a null data packet announcement (NDP-A) frame including information on transmission of a subsequent null data packet (NDP) frame, and transmits an NDP frame to the at least one station on the basis of information on the NDP-A frame. Further, the AP receives, from the at least one STA, a feedback frame including downlink channel quality information. At this time, the NDP frame includes an L-part for a legacy STA and a high efficiency (HE)-part for a HE STA, wherein the HE-part consists of a HE-signaling A (HE-SIG A) field, a HE-short training field (HE-STF), and a HE-long training field (HE-LTF).