1. A brain disease diagnosis system for diagnosing a brain disease of an examined person, comprising: an imaging modality configured to capture a brain image of the examined person, the imaging modality being selected from the group consisting of a CT machine, an MRI machine, a PET machine, and a SPECT machine; a gateway server including a CPU and a memory for receiving the brain image from the imaging modality and accommodating the brain image of the examined person; and a server including a CPU and a memory configured to execute the following processes, acquire the brain image from the memory of the gateway server; set a plurality of regions in the brain image; calculate an individual index value based on a pixel value of the brain image for each of the plurality of regions set in the brain image; calculate both sensitivity and specificity for each of the plurality of regions set in the brain image; calculate a weight coefficient based on a sum of the sensitivity and specificity for each of the plurality of regions set in the brain image; calculate a whole index value by (i) multiplying the calculated weight coefficient and the individual index value for each of the plurality of regions set in the brain image by one another, thereby determining a product value for each of the plurality of regions set in the brain image; and then (ii) adding the product values resulting from step (i), thereby yielding a total value which is the calculated whole index value; and diagnose a brain disease of the examined person based on the calculated whole index value.