Disclosed herein is a method of transmitting a UL signal from a user equipment (UE) in a wireless communication system supporting an unlicensed band and apparatuses for supporting the same. More specifically, the present invention provides an embodiment in which the UE performs autonomous uplink transmission and scheduled uplink transmission through the unlicensed band, a method of adjusting contention window size when the UE perform the autonomous uplink transmission through the unlicensed band, and an embodiment of performing the autonomous uplink transmission based on the method.