Provided is a technique for computing a brain function index for diagnosing a mental disorder at the individual level, which can be applied to even a child around school age. A brain function index computing device includes a brain activity value calculation processing unit configured to calculate from brain activity signals of a subject a first brain activity value of a region including the meddle frontal gyrus (MFG) and a second brain activity value of a region including the inferior frontal gyrus (IFG) and a brain function index calculation processing unit configured to calculate a brain function index associated with a mental disorder using the first brain activity value and the second brain activity value.
