1. A magnetic resonance diagnostic apparatus comprising: a derivation unit configured to derive an apparent diffusion coefficient based on pixel values of respectively corresponding a pixel positions included in a region of interest for each of at least two original images obtained by imaging a same imaging region of a same subject using at least two respectively corresponding different b-factors; a first estimation unit configured to estimate a pixel value that would have been obtained by using a b-factor which is larger than the at least two b-factors, for each pixel position included in the region of interest, based on the apparent diffusion coefficient derived for each pixel position; an imaging unit configured to acquire the at least two original images; and a second estimation unit configured to estimate signal-to-noise ratio (SNR) in a first original image acquired using a first b-factor b b , based on a signal value S(b) in the first image and signal value S(a) and noise level in a second original image which is one of the at least two original images, and a diffusion coefficient D set in advance for a predetermined portion of the subject positioned in the imaging region, wherein the imaging unit is configured to acquire the first original image using a b-factor same as the first b-factor in a case where the SNR estimated by the second estimation unit is equal to or higher than a predetermined threshold value, and to acquire the second original image using a b-factor different from the first b-factor in a case where the SNR estimated by the second estimation unit is lower than the threshold value.