Disclosed is a radar signal processing device 2 including frequency domain converters 201-1 to 201-M to convert raw data 11-1 to 11-M showing observation results acquired by a radar device 1 into those in a frequency domain, a signal restorer 202 to synthesize the raw data 11-1 to 11-M whose domain is converted into the frequency domain according to a least square method, a time domain converter 203 to return a signal series after synthesis to that in a time domain, and an image reproducer 204 to acquire a reproduced image 12 by performing image reproduction on the signal series whose domain is returned to the time domain.