1. A magnetic resonance diagnostic apparatus comprising: MRI gantry components comprising static and gradient magnetic field generators, at least one radio frequency (RF) coil coupled to an imaging volume, transmitter and receiver and computer circuits connected to control said gradient magnetic field generator, said transmitter and said receiver, said computer circuits being configured to: obtain at least two original MR images by imaging, using said MRI gantry components, an imaging region on a subject using at least two b-factors, respectively; 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 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; and generate a calculated image by arranging the estimated pixel values with respect to each of the pixel positions; wherein the at least two b-factors are set in accordance with a portion of the subject included in the imaging region.