The time spent in handover is shortened when a mobile station apparatus performs random access for handover. In a mobile communication system where a mobile station apparatus 200 uses any signature of a beforehand determined signature group between the mobile station apparatus 200 and a base station apparatus 100 in random access, the base station apparatus 100 selects a signature used by the mobile station apparatus 200 in random access performed at the time of handover, and the mobile station apparatus 200 performs handover using the signature selected in the base station apparatus 100 and judges the presence or absence of transmission and reception of a handover complete message and contention resolution corresponding to the type of signature selected in the base station apparatus 100.