The present invention provides: a pharmaceutical composition for inhibiting amyloid-² protein accumulation comprising a compound of the formula (I) or a pharmaceutically acceptable salt thereof as an active ingredient； a method for inhibiting amyloid-² protein accumulation, comprising a step of administering an effective amount of the compound of the formula (I) or a pharmaceutically acceptable salt thereof to a mammal which may be diagnosed with an amyloid-² protein-related disease； and so on.
