The purpose of the present invention is to provide a tilt angle deciding method and a system wherein tilt angles to diminish the deterioration rate of the entire system can be obtained in the radio communication system. The method comprises the steps of setting initial values of tilt angles; calculating the deterioration rate at the initial values of tilt angles; selecting an antenna for reducing a tilt angle; reducing the tilt angle of the selected antenna; calculating a deterioration rate at the time when the tilt angle is reduced; determining the continuity of repeating the processes, after repeating the series of processes for reducing these tilt angles, and increasing tilt angles repeatedly in the similar way. In addition, the method comprises the steps of outputting tilt angles for diminishing a small deterioration rate, and determining whether the processes of repeating the whole steps finish. With the above-specified processes, it becomes possible to obtain tilt angles for diminishing the deterioration rate of the entire system in comparison with that obtained at initial values of tilt angles.