Provided are a radio communication base station device, a radio communication terminal device, and a radio communication system capable of reducing the processing load of a base station and an access gateway in a cell reselection process. When desynchronization between a terminal (150) and a source base station is detected, the terminal (150) performs a narrow-region cell search for an adjacent base station connected directly to the source base station. If no adjacent base station is detected, a wide-region cell search is performed for other than the adjacent base station. A base station detected is made as a target base station and a cell update is requested. The source base station transmits terminal information to the access gateway after a timer has counted up the time required for the narrow-region cell search at the terminal. The base station which has received the cell update request acquires a terminal acknowledgment and terminal information from the source base station or the access gateway depending on whether the source base station is directly connected.