A method and device for transmitting data in a wireless LAN system are provided. Particularly, a transmission device receives configuration information relating to a multi-band. The transmission device performs channel sensing with respect to the multi-band. The transmission device transmits data to a reception device via the multi-band on the basis of the result of the channel sensing. A first band and a second band are combined in the multi-band. The first band comprises a first primary channel, and the second band comprises a second primary channel. The result of the channel sensing is obtained on the basis of whether or not the first primary channel is idle until a first BC value becomes 0 and whether or not the second primary channel is idle until a second BC value becomes 0. The first BC value is selected within a first CW configured with respect to the first primary channel. The second BC value is selected within a second CW configured with respect to the second primary channel.