In a WCDMA mobile communication system, when the amount of packet traffic transmitted to or received from a UE decreases during a predetermined time in a CELL_DCH state in which a WCDMA packet data connection has been established, the UE transitions to a CELL_FACH state by a radio network controller (RNC). Thereafter, it is determined if there is no transmitted or received packet traffic during a certain time in the CELL_FACH state (which is an RRC state in which an RRC connection has been connected, and the dedicated physical channel has been released). When it is determined that there is no transmitted or received packet traffic during the certain time, the UE requests a network to release the RRC connection by utilizing a Signaling Connection Release Indication (SCRI) message. The invention enables a UE to check a packet transmission or reception flow, and to directly transmit the SCRI message to the network.