When the terminal satisfies a given condition, the terminal may determine an SN change, and determine, as a target cell, a candidate cell related to the condition (operation 9-60). The terminal performs a random access to a target cell of a corresponding target SN (operation 9-90). In addition, the terminal may transmit an MN RRC message (e.g., an RRC connection reconfiguration complete message or ULInformationTransferMRDC) to the MN so as to notify the MN that a conditional PSCell is performed (operation 9-70). As a message included in the MN RRC message, an SN RRC reconfigurationcomplete message or a measurement report message may be used. Together with the message, information on a configuration ID for conditional PSCell change transferred from the MN in operation 9-50. When an MR is included, a measurement ID, the MR of which has been triggered, may indicate one of previously given conditions configured for conditional PSCell change/addition, and the S-SN may notify, through the MR, a particular target PSCell that a conditional change is performed. Any type of MN RRC/SN RRC messages may be transferred through an srb1 (or split srb1). Operation 9-70 and operation 9-90 may be sequentially performed or performed in parallel. The MN RRC reconfiguration complete message includes an SN RRCreconfigurationComplete message of the SN in an octet string type. The SN RRCreconfigurationcomplete message may be made as a response message for an SN RRC message including sCHO configuration information of a target cell of a particular T-SN selected for performing of sCHO in operation 9-60 by using a transaction ID of the SN RRC message. The MN may identify a target cell and a target node for which sCHO is performed, through an SN RRC reconfiguration complete message included in an MN RRC reconfiguration complete message, or may identify the same cell and node when the MN receives an MN RRCreconfigurationcomplete message transmitting a candidate ID or PCI/CGI information of a cell selected as a target, or a conditional reconfiguration ID value assigned to each candidate target PSCell of a conditional PSCell change assigned by the MN in operation 9-50. The MN having received the message (transaction ID or selected cell ID) of operation 9-70 may transmit an SgNB release request message to the S-SN (operation 9-75), and receive an SgNB release request ack message from the S-SN (operation 9-80). Thereafter, the MN may recognize a particular t-SN, based on a transaction ID of a selected T-SN, or when information on an ID of a T-SN selected by the terminal as a target and a target cell ID (candidate ID, or information of PCI or CGI) is included in an RRCreconfigurationcomplete message received in operation 9-70, the MN may recognize a particular t-SN, based on this information, and transmit an SgNBReconfigurationComplete message to the corresponding selected t-SN (operation 9-85). The SgNB reconfiguration complete message may include an NR complete message received from the terminal.