Disclosed herein is a sounding method of a station (STA) for sending feedback information about a downlink (DL) channel state in a wireless LAN (WLAN) system. The sounding method includes receiving an NDP announcement (NDPA) frame providing notification of the transmission of a null data packet (NDP) frame; receiving the NDP frame; receiving a trigger frame comprising resource allocation information allocated to the STA; generating feedback information using the NDP frame; and sending a feedback frame comprising feedback information through uplink (UL) multi-user (MU) transmission using a frequency resource indicated by the resource allocation information. The feedback information may be generated in a frequency granularity (FG) unit with respect to a transmission bandwidth having a size smaller than or equal to the size of a transmission bandwidth of the NDP frame.