The present invention relates to a wireless communication system and, more particularly, to a method and an apparatus therefor, the method comprising the steps of: identifying a minimum storing area per data in a HARQ buffer on the basis of a TTI length; storing data for transmission of a wireless signal in the HARQ buffer on the basis of the minimum storing area per data; and transmitting the data in the HARQ buffer during a first TTI, wherein, when the data is retransmitted data, the minimum storing area per data is based on the length of a second TTI used for initial transmission of the data, and the length of the second TTI is different from the length of the first TTI.