The base station notifies each mobile station of reporting conditions for causing the mobile station to transmit a measurement report containing measurement results of the radio quality. The reporting conditions include a first reporting condition and a second reporting condition in which the offset value added to the measurement result of the radio quality is greater than that of the first reporting condition and in which the guard time, that is a time from when the reporting condition is satisfied until the measurement report is transmitted, is shorter than that of the first reporting condition. The base station detects a mobile station that has transmitted a measurement report as a result of satisfaction of the second reporting condition as a handover required of mobile station whose radio quality becomes lower than a predetermined value within a predetermined period and performs a handover process of the mobile station.