A radar signal processing device is provided, which performs a scan correlation in a polar coordinate system to secure accuracy of the scan correlation, and prevents a suppression of a target object moving at high speed due to the scan correlation. A polar coordinate correlator performs, in a polar coordinate system, a correlation between reception data and previous correlated data stored in a previous data storage. A trend curve calculating module calculates a trend curve of a distance-direction signal level of the reception data in the polar coordinate system. A target detecting module detects a target based on the signal level of the reception data and the trend curve. Further, the polar coordinate correlator changes the contents of the correlation of the reception data based on the target detection result from the target detecting module.