A tau aggregation inhibitor can sufficiently inhibit a tau protein from aggregating in cells. The inhibitor includes a 4-substituted catechol structure compound, having, at position 4 of its catechol ring, an electron-donating substituent R other than a hydrocarbon group, or a salt thereof. The 4-substituted catechol structure compound is preferably 4-aminocatechol or 1,2,4-benzenetriol. Examples of tauopathies to which this inhibitor is applicable include AD, Down's syndrome, frontotemporal dementia, cotricobasal degeneration (CBD) and progressive supranuclear palsy (PSP).