A mobile communication system includes at least one upper radio station (1) and a relay station (2). The relay station (2) performs a radio relay between a lower radio station (3) and the upper radio station (1). The relay station (2) is configured to attach, when an attaching to a first upper radio station included in attaching configuration information indicating an upper radio station to which the relay station (2) should attach is unsuccessful, to a second upper radio station different from the first upper radio station, and to notify a network (4 and 5) of notification information via the second upper radio station, the notification information indicating that the attaching to the first upper radio station is unsuccessful.