The present document relates to a method for transmitting an acknowledgment/negative-acknowledgment (ACK/NACK) signal for data transmitted from a plurality of stations (STAs) by an access point (AP) in a wireless LAN (WLAN) system and an apparatus therefor. To this end, the AP transmits a trigger frame to the plurality of STAs, receives data transmitted from the plurality of STAs in response to the trigger frame, and transmits and ACK/ANCK signal for the data received from the plurality of STAs. In this process, when all the data received from the plurality of STAs have been successfully received, the ACK/NACK signal may be transmitted not using a block ACK frame but using an ACK frame.