1. A system for improving brain function in order to prevent dementia, the system comprising: a brain-function-improving content storage unit that subdivides content for judging a degree of activation of human brain function into multiple classified regions and levels and stores the subdivided content; a content selection unit that selects the content stored in the brain-function-improving content storage unit for each region according to a predetermined classification method; and an achievement determination unit that determines achievement of a user based on scores acquired through feedback provided by the user for the content selected by the content selection unit.