The invention relates to a method for acquiring, by use of a continuous-wave electron spin resonance imaging scheme that sweeps a magnetic field, a 14N nitroxy radical distribution image and a 15N nitroxy radical distribution image in a single image capturing operation. This image acquiring method is characterized in that the central one (B) of three absorption peaks derived from 14N nitroxy radical is caused to move to a higher or lower magnetic field side and Fourier transform is used to determine a parameter (α), which represents the amplitude of the absorption peak of the higher or lower magnetic field side, and a parameter (γ) which represents the peak position of that absorption peak.