The present invention provides: a pharmaceutical composition for inhibiting amyloid-β protein accumulation comprising a compound of the formula (I) or a pharmaceutically acceptable salt thereof as an active ingredient; a method for inhibiting amyloid-β 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-β protein-related disease; and so on.