1. A method for assisting in the diagnosis of cerebral diseases, wherein brain images of one subject are inputted and subjected to image processing to output the diagnosis, result of the subject, thereby assisting in the diagnosis, the method comprising the steps of: determining an assembly of voxels on a standardized brain image as a region of interest based upon a statistically significant difference detected between two image groups, wherein one image group is of multiple diseased patients and another image group is of multiple non-diseased patients for a certain disease, and retaining them; statistically comparing the thus input brain images of the subject with the previously prepared brain images of normal subjects by applying the region of interest for a disease to be diagnosed on the standardized brain image, and calculating values as several indexes that show the comparison result; displaying the statistical comparison result as diagnosis assisting information, wherein the statistical comparison is made by calculating Z scores for every voxel, a limited mean value of Z scores is calculated among voxels of the one subject that have Z scores over a predetermined value and that also belong to the region of interest for the disease to be diagnosed, and the limited mean value is included in the statistical comparison result.