A method and a device for transmitting data in a wireless communication system are discussed. The method according to an embodiment includes transmitting a plurality of request to send (RTS) frames through a plurality of subchannels, each of the plurality of subchannels having a 20 megahertz (MHz) bandwidth; receiving at least one clear to send (CTS) frame through at least one idle subchannel; and transmitting a data frame. If the at least one CTS frame includes channel information related to the at least one idle subchannel through which the CTS frame is transmitted by the station, the data frame is transmitted through the at least one idle subchannel indicated by the channel information. A number of the at least one idle subchannel through which the CTS frame is transmitted is less than a number of the plurality of subchannels through which the plurality of RTS frames are transmitted.