A method for detecting an occurrence of interference between a return of a radar wave which has been transmitted by a radar and has an oscillating amplitude in time and a radio wave transmitted by some other radar, Includes 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, 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 that the interference occurs during the detected period of time.