Disclosed are a method and a device by which an access point (AP) transmits a block ACK (BA) signal for transmission data of a plurality of stations (STAs) in a wireless LAN (WLAN) system. To this end, an AP receives data from a plurality of STAs, and transmits a first multi-STA BA frame (M-BA frame) for the data received from the plurality of STAs, wherein the first M-BA frame includes a MAC header field, a BA control field, and a BA information field, a last N bit of the BA control field indicates at least one from among the number of pieces of BA information included in the first M-BA frame and information on a succeeding ACK frame, and N is set to 5 or more.