Disclosed are a radio communication terminal device for reducing a power consumption needed for releasing an RRC Connection after a position registration, a network device, a mobile communication system and a position registering method. In this position registering method, in case the RRC state of a UE is in an Idle state at ST501, the UE sets flag=1, whereas the RRC Connection is established between the UE and a UTRAN. The flag set at ST501 is sent at ST504 to the UTRAN, and the flag received by the UTRAN is stored at ST505. At ST507, an RA Update Accept received by the UTRAN is sent to the UE, and the RRC Connection is released in case the value of the flag stored at ST505 is 1. When the UE receives a DOWNLINK DIRECT TRANSFER, on the other hand, the RRC Connection is released in case the value of the flag set at ST501 is 1.