A method of stabilizing lipid-rich plaques and method of preventing the rupture thereof, characterized in that an effective amount of 2-[4-[2-(benzimidazol-2-ylthio)ethyl]piperazin-1-yl]-N-[2,4-bis(methylthio)-6-methyl-3-pyridyl]acetamide, its acid adduct salt or a hydrate thereof is administered to patients with lipid-rich plaques. Prevention of plaque rupture and stabilization of plaques can be attained by reducing the occupancy of macrophages in lipid-rich plaques being unstable and tending to rupture among plaques being a lesion from a seat of atherosclerosis and simultaneously increasing the occupancy of collagen therein.