Provided is a reference data converting section that rearranges each of a plurality of frequency components along the frequency axis such that (i) a fundamental wave component and harmonic wave components of a reference digital signal are within a first Nyquist region of a spectrum of the reference digital signal and (ii) image components of the fundamental wave component and the harmonic wave components are within a second Nyquist region of the spectrum; and a distortion detecting section that detects the non-linear distortion of the reference digital signal caused by each harmonic wave component, based on each harmonic wave component having a prescribed order number in the spectrum in which each frequency component has been rearranged by the reference data converting section.