According to one embodiment of the present invention, a method by which a station (STA) performs an extended inter-frame space (EIFS) operation in a wireless LAN system supporting multi-user (MU) transmission comprise the steps of: receiving a MU frame including a SIG-A field and a SIG-B field; estimating, by other STAs for receiving the MU frame, an ACK transmission time (ACK Tx Time) required for transmitting ACK information when the MU frame causes the EIFS operation; and delaying channel access during an EIFS time determined on the basis of the estimated ACK transmission time, wherein the step for estimating the ACK transmission time enables the estimation of the ACK transmission time by assuming the lowest MCS level among modulation and coding scheme (MCS) levels used for the transmission of the ACK information, and the minimum resource (RU) among the RUs used for the transmission of the ACK information.