Provided is a technique for calculating an oxygen extraction fraction by using MRI where the oxygen extraction fraction in a brain including brain parenchyma is calculated via a simple processing without an impact on an examinee, such as administration of caffeine. For this purpose, an MRI apparatus of the present invention measures a complex image of nuclear magnetic resonance signals, and calculates from thus measured complex image, a physical property distribution for obtaining a physical property image reflecting the oxygen extraction fraction. Then, thus calculated physical property distribution is separated into tissue-specific physical property distributions for at least two tissues (separated tissue images). After converting any of the separated tissue images into the oxygen extraction fraction, a distribution of the oxygen extraction fraction is estimated based on the condition that a value of any selected pixel is substantially equal to a mean value of pixels surrounding the selected pixel.