1. A magnetic resonance diagnostic apparatus comprising: an imaging unit to obtain at least two original images by imaging an imaging region on a subject using at least two b-factors, respectively; a derivation unit to derive an apparent diffusion coefficient regarding a pixel position for each pixel position included in a region of interest in the at least two original images, based on pixel values of each of the at least two original images regarding the pixel positions; and an estimation unit to estimate a pixel value obtained by using a b-factor which is different from the at least two b-factors, regarding each pixel position included in the region of interest, based on the apparent diffusion coefficient derived for each pixel position; wherein the at least two b-factors are set in accordance with a portion a the subject included in the imaging region.