A terminal may transmit a measurement report to a T-SN or an S-SN (operation 9-05). The terminal may transmit a measurement report to an MN through a signaling radio bearer (SRB)1, and the MN may transfer the measurement report received from the terminal to an SN, or the terminal may directly transmit a measurement report to an SN by using an SRB3. The source SN (S-SN) may determine a conditional PSCell change (operation 9-10). The S-SN may determine to perform a conditional PSCell change (sCHO), based on the received measurement report, determine candidate cells, and determine a performing condition for each candidate cell. According to the determination of operation 9-10, the S-SN may transfer a base station change required message (e.g., SgNB change required message) to the MN (operation 9-15). The SgNB change required message may include at least one of a determined sCHO indication, an sCHO candidate cell list, a condition for each candidate cell, source cell group-configuration (sCG-Config) information including a current source configuration of the terminal, or source cell configuration information, and a target cell group-configuration (tCG-Config) that is a message made by the S-SN and including condition information. The current source configuration information may indicate source cell information that is currently applied to the terminal, or source cell information that the terminal receives from the S-SN and applies when a reconfiguration message of operation 9-50 is received. The MN having received the SgNB change required message and information included in same transmits a base station addition request message (e.g., SgNBAdditionRequest message) to each T-SN (operation 9-20). Information contained in this message may include at least one of an sCHO indicator, sCG-Config received from the S-SN, tCG-config, a target candidate ID list and a target candidate cell ID (PCI) list associated therewith, and CG-ConfigInfo that is terminal configuration information required by the MN from a corresponding T-SN. When the SgNB addition request message is received by T-SN1, T-SN1 may perform an admission control for a given candidate cell so as to determine an available candidate cell (operation 9-25). When T-SN1 determines to refuse a particular candidate cell, T-SN1 may decode received tCG-Config, that is, an SN RRC reconfiguration message made by the S-SN, and erase all information of conditions and candidate cell configurations associated with cells for which T-SN1 fails in admission. T-SN1 may encode the message again, add changed tCG-Config information in an SgNBAddition reject message, and transmit same to the MN (operation 9-30). Regarding an operation of a case where T-SN1 determines to allow a candidate cell, operation 9-45 is referred to. The MN may sequentially transmit an SgNBAdditionRequest message to T-SN2 that is another T-SN (operation 9-35). In addition, the MN may also transmit SgNB Addition request messages to different T-SNs in parallel. This message may include at least one of an sCHO indicator, sCG-Config, updated tCG-Config previously received from T-SN1, CG-ConfigInfo, and cell identification information including a candidate cell ID list included in T-SN2. T-SN2 having received the SgNB addition request message may perform an admission control to perform the admission control for a given candidate cell and determine an available candidate cell (operation 940). When T-SN2 determines to accept a particular candidate cell, T-SN2 may associate, with candidate cell configuration information made by T-SN2, an entry of a relevant condition included in an ID of an accepted candidate cell or an sCHO reconfiguration ID of the accepted candidate cell (this ID includes configuration and condition information for each sCHO candidate cell and indicates same) for an SN RRC reconfig message made by the S-SN (when a sequential SN addition request is requested, this message may indicate an updated tCG-Config message received from a previous T-SN, and when a parallel SN addition request is requested, this message may indicate a tCG-Config message received in operation 9-15), and add same to tCG-Config. The additional operation may correspond to an operation in which T-SN2 decodes a received tCG-Config message, and adds a configuration part made by T-SN2 for an sCHO candidate cell accepted by T-SN2 in an sCHO reconfiguration ID list (i.e., scho-ToAddModList) contained in the corresponding message according to a result of an admission control, and the additional operation is not limited thereto (for example, include the T-SN generated part in the octet string defined for the concerned sCHO candidate within scho-ToAddModList). This configuration information may be delta configuration information based on configuration information of the S-SN. A case where the T-SN refuses sCHO may correspond to an operation in which T-SN2 removes sCHO refused by T-SN2 from an sCHO reconfiguration ID list contained in tCG-Config (for example, remove the sCHO from the scho-ToAddModList). T-SN2 may encode again a resulting configuration obtained through addition or removal by T-SN2 to make an RRC message, and this message may correspond to updated tCG-Config.