Proposed are a method and a device for transmitting data in a wireless LAN system. Specifically, a transmission device receives configuration information of a multi-band formed by aggregating first to third bands. The transmission device carries out channel sensing on the first to third bands. The transmission device transmits data to a receiving device on the basis of the result of the channel sensing. The first band comprises a first primary channel and a first secondary channel; the second band comprises a second primary channel and a second secondary channel; and the third band comprises a third primary channel and a third secondary channel.