1. A magnetic resonance diagnostic apparatus comprising: a derivation unit derive an apparent diffusion coefficient regarding a pixel position for each pixel position included in a region of interest in at least two original images obtained by imaging a same imaging region of a same subject using at least two b-factors which are different from each other, respectively, based on pixel values of each of at least two original images regarding the pixel positions; and a first 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.