PROBLEM TO BE SOLVED: To obtain the subject inhibitor having excellent β-amyloid protein production/secretion inhibitory activity, therefore useful as a preventive/ therapeutic agent for diseases including senile dementia, Alzheimer's disease, Down's syndrome and Parkinson's disease, by making the inhibitor include a compound having a specific monocyclic aromatic ring. SOLUTION: This inhibitor is obtained by including a compound of the formula [Ar is a (substituted) aromatic group; X and Y are each O, S, or the like; R1 and R2 are each H or a (substituted) 1-6C alkyl; A-ring is a (substituted) monocyclic aromatic ring] or a salt thereof, e.g. 5-chloro-N-[2-(N,N-diethylamino) ethyl]-2-methoxy-4-(1-naphthoylamino)benzamide; the salt of this compound is e.g. a sodium salt, salt with triethylamine or the like. The content of the compound of the formula in the corresponding pharmaceutical composition is 0.1-100 wt.%. The dose of this inhibitor as an oral agent is pref. 5-100 mg/ adult at a time.