1. A method used by a radar for detecting an occurrence of interference between a return of a radar wave which has been transmitted by the radar and has an oscillating amplitude in time and a radio wave transmitted by some other radar, comprising steps of: detecting extremal points of an incident radio wave in which the radio wave transmitted by the other radar is superposed on the return of the radar wave, each of the extremal points being a moment at which either a maximum or a minimum in amplitude of the incident radio wave appears; extracting, sequentially in time, an emerging pattern of the extremal points of the incident radio wave within each of periods of time to obtain a series of emerging patterns of the extremal points; detecting a period during which the emerging pattern of the extremal points is irregular among the series of the emerging patterns of the extremal points; and determining if the interference has been detected as occurring within the detected period; wherein the emerging pattern is quantified by a number of the extremal points of the incident radio wave at a predetermined count period to determine a series of counted numbers of the extremal points of the incident radio wave, and the step of detecting the period during which the emerging pattern of the extremal points is irregular comprises: averaging the counted numbers of the extremal points of the incident radio wave to determine an average number of the extremal points per counting period; and detecting the period during which a difference between the average number of the extremal points and one of the counted numbers of the extremal points of the incident radio wave within the one of the counting periods exceeds a predetermined threshold.