There is provided a coherence length measurement device (10) that calculates a coherence length Lc based on a spectrum v calculated by an FFT device (9) and performs, in case where the coherence length Lc is shorter than an FFT gate width Gw, a setting change to shorten the FFT gate width Gw and a pulse width Pw, and the FFT device (9) performs frequency analysis on a received signal outputted from an A/D converter (8) by a unit of an FFT gate following the setting change to calculate the spectrum v of the received signal.