The present specification relates to a method of an access point (AP) transmitting a block acknowledgement (ACK) signal with respect to transmission data of a plurality of stations (STA) in a wireless LAN (WLAN) system, and an apparatus therefor. To this end, an AP transmits a multi-STA BA frame with respect to data received from the plurality of STA, wherein the multi-STA BA frame includes a MAC header field, a BA control field and a BA information field. In this regard, the BA information field may include: AID information with respect to one or more STA to which ACK/NACK is transmitted from the plurality of STA; and an individual information field including BA indicator information that indicates whether the BA information field includes one or more of a block ACK bitmap sub-field and a starting sequence control sub-field.