A tau aggregation inhibitor that can sufficiently reduce tau aggregation in cells is provided. The tau aggregation inhibitor includes a catechol structure-containing compound or a salt thereof, and the catechol structure-containing compound is selected from the group consisting of isoprenaline, dopamine, dobutamine, levodopa, levodopa/carbidopa, trimetoquinol, hexoprenaline, methyldopa, and droxidopa. The catechol structure-containing compound is preferably isoprenaline. Tauopathies to be prevented or treated by the inhibitor include AD, Down's syndrome, Pick's disease, corticobasal degeneration (CBD), and progressive supranuclear palsy (PSP).