A phase distribution measurement method inside a biological sample includes taking in an optical image of the sample formed by a microscope to form a plurality of images with different image contrasts; calculating a component corresponding to phase distribution of the sample and a component corresponding to other than the phase distribution, and dividing the component corresponding to the phase distribution by the component corresponding to other than the phase distribution to forma normalized phase component image; breaking down the phase component image into a plurality of frequency components; performing a deconvolution process to each of the frequency components using an optical response character corresponding to each, and calculating phase distribution of a refraction component and phase distribution of a structure component; and calculating phase distribution of the sample by compounding the phase distribution of the refraction component and the phase distribution of the structure component.