An ACK signal processing method that can be used in a wireless LAN system is proposed. Various information that can be used for a downlink MU PPDU for multiple receiving stations may be proposed, and, for example, in order to omit the re-transmission of the related art, information requesting an ACK or Block ACK corresponding to previously transmitted data may be included. If an immediate acknowledgement (ACK) frame fails to be received after the MU-PPDU is transmitted from the AP, a Block ACK may be requested. In this case, while a Block ACK is requested, a radio resource through which a Block ACK is delivered may be allocated.