A method for performing backoff in a Wireless Local Area Network, WLAN, system, the method comprising: performing (S2610, S2620) a backoff process; and transmitting (S2630, S2640) a Physical Layer Protocol Data Unit, PPDU, according to a result of the backoff process; characterized in that a primary 2 MHz channel is comprised of a primary 1 MHz channel and a secondary 1 MHz channel, i) when transmission of a first PPDU having 1 MHz width is intended, the backoff process is performed on the primary 1 MHz channel, using a first clear channel assessment, CCA, condition, and the transmission of the first PPDU is allowed only at the primary 1 MHz channel without allowing use of the secondary 1 MHz channel when a backoff timer corresponding to the primary 1 MHz channel reaches 0; and ii) when transmission of a second PPDU having 2 MHz, 4 MHz, 8 MHz or 16 MHz width is intended, the backoff process is performed on the primary 2 MHz channel, using a second CCA condition which is different from the first CCA condition, wherein a back-off timer corresponding to the primary 2 MHz channel freezes while the primary 1 MHz channel is busy, and the second PPDU is transmitted after the back-off timer corresponding to the primary 2 MHz channel reaches 0. performing (S2610, S2620) a backoff process; and transmitting (S2630, S2640) a Physical Layer Protocol Data Unit, PPDU, according to a result of the backoff process; characterized in that a primary 2 MHz channel is comprised of a primary 1 MHz channel and a secondary 1 MHz channel, i) when transmission of a first PPDU having 1 MHz width is intended, the backoff process is performed on the primary 1 MHz channel, using a first clear channel assessment, CCA, condition, and the transmission of the first PPDU is allowed only at the primary 1 MHz channel without allowing use of the secondary 1 MHz channel when a backoff timer corresponding to the primary 1 MHz channel reaches 0; and ii) when transmission of a second PPDU having 2 MHz, 4 MHz, 8 MHz or 16 MHz width is intended, the backoff process is performed on the primary 2 MHz channel, using a second CCA condition which is different from the first CCA condition, wherein a back-off timer corresponding to the primary 2 MHz channel freezes while the primary 1 MHz channel is busy, and the second PPDU is transmitted after the back-off timer corresponding to the primary 2 MHz channel reaches 0. i) when transmission of a first PPDU having 1 MHz width is intended, the backoff process is performed on the primary 1 MHz channel, using a first clear channel assessment, CCA, condition, and the transmission of the first PPDU is allowed only at the primary 1 MHz channel without allowing use of the secondary 1 MHz channel when a backoff timer corresponding to the primary 1 MHz channel reaches 0; and ii) when transmission of a second PPDU having 2 MHz, 4 MHz, 8 MHz or 16 MHz width is intended, the backoff process is performed on the primary 2 MHz channel, using a second CCA condition which is different from the first CCA condition, wherein a back-off timer corresponding to the primary 2 MHz channel freezes while the primary 1 MHz channel is busy, and the second PPDU is transmitted after the back-off timer corresponding to the primary 2 MHz channel reaches 0.