The present invention relates to a method, an apparatus and a system for adjusting a contention window size for performing channel access. In particular, the present invention provides a method comprising: receiving a plurality of hybrid automatic repeat request acknowledgment (HARQ-ACK) responses for downlink transmission(s) in a reference subframe of the specific cell; setting a contention window size for a transmission in the specific cell; selecting a random number N within the contention window size; and performing, when a channel of the specific cell is idle for at least N slot periods, a downlink transmission on the channel, wherein the setting a contention window size comprises: increasing the contention window size to a next higher allowed value and maintaining the increased value when a ratio of negative acknowledgement (NACK) among the plurality of HARQ-ACK responses is equal to or more than a reference value, and setting the contention window size to a minimum value when the ratio of the NACK among the plurality of HARQ-ACK responses is less than the reference value, and an apparatus and a system therefor.