The present invention relates to a wireless communication system and, more specifically, to a method and a device therefor, the method comprising the steps of: receiving, as a part of an RRC connection procedure, CORESET configuration information relating to a BWP, the CORESET configuration information including information relating to a frequency resource configuring a CORESET; monitoring PDCCH candidates in one or more CORESETs in the BWP; and detecting a PDCCH on the basis of the monitoring, wherein the same CORESET is configured for each of subbands in the BWP according to the frequency resource, on the basis of (i) the BWP including a plurality of subbands, and (ii) the frequency resource confined within one subband.