The present disclosure relates to a wireless communication system and, more particularly, to a method and an apparatus therefor, the method comprising the steps of: monitoring first PDCCH candidates for a second cell in a first SS of a first cell; on the basis of the second cell being PCell, monitoring second PDCCH candidates for the second cell even in a second SS of the second cell; and on the basis of a PDCCH detected from among the first and second PDCCH candidates, performing transmission or reception of data corresponding to the PDCCH.