1. A medical image display processing method comprising the steps of: inputting a functional image in an ASL coordinate system from a head of a subject by an ASL imaging method using an MRI device; subjecting the inputted functional image to mask processing to extract only a region of cerebral parenchyma; and displaying the resultant functional image containing only the extracted region of the cerebral parenchyma, wherein the mask processing for extracting only the region of the cerebral parenchyma is performed using a mask image produced from an ASL-EPI image inputted simultaneously by the ASL imaging method, an MRI morphological image is inputted from the head of the subject using the MRI device, transformation field from ASL coordinate system of the subject to MRI coordinate system is determined based on the inputted MRI morphological image and the ASL-EPI image, the functional image of only the region of the cerebral parenchyma in the ASL coordinate system is transformed into an image in the MRI coordinate system using the transformation field, and the resultant functional image of the cerebral parenchyma is displayed to be overlaid on the MRI morphological image.