A handover decision is capable of lowering a handover failure rate of horizontal handover between a source base station and a target base station in a wireless communication system. After deciding a target base station to which the mobile terminal is to be handed over, a hysteresis value for determining a handover execution point is decided, and execution of the handover of the mobile terminal occurs when a difference between the received signal strength of a signal received from the target base station to the mobile terminal and the received signal strength of a signal received from the source base station to the mobile terminal is continuously maintained at the hysteresis value or above during a predetermined time.