A method includes inputting an echo signal from an antenna for discharging an electromagnetic wave to a predetermined area and receiving an echo signal reflected on a target object, outputting a level of the echo signal from every location so as to associate the level with a distance from the antenna in an azimuth direction where the electromagnetic wave is discharged, calculating a degree of change in the level of the echo signal from mutually adjacent locations among all the locations concerned, performing edge emphasis processing for the level of the echo signal in the azimuth direction based on the degree of change and outputting an edge-emphasized echo signal, and performing scan-to-scan correlation processing to calculate a correlation value of the echo signals of two or more scans using the edge-emphasized echo signals, where the echo signals from the entire predetermined area is used as one scan.