1. A radar return signal processing apparatus comprising: a detector which detects an average Doppler frequency, a spectrum width and a received power of each of echoes included in a radar return signal, from the radar return signal obtained repeatedly at regular intervals and detects a frequency spectrum based on the average Doppler frequency, the spectrum width and the received power; an estimation unit which estimates an optimum mixed density function by modeling a shape of the frequency spectrum by calculating repeatedly a sum of density functions of each of the echoes; and an extraction unit which extracts information on any one of the echoes included in the radar return signal, from a parameter of the estimated mixed density function.