In a first standby state, the reception quality of a signal from a system which has established synchronization is detected. If the detected reception quality deteriorates to be less than a threshold value, the destination of synchronization establishment is changed from a first system to a second system. In a second standby state after this system change, the reception quality of a system having a higher priority order of selection than the second system which has established synchronization is detected. If a third system whose detected reception quality exceeds the threshold value is found, the destination of synchronization establishment is changed to this third system again. After that, the first standby state is returned.