The present invention relates to nanoparticles containing pioglitazone or a salt thereof and a biocompatible polymer, a pharmaceutical preparation containing the nanoparticles for the prophylaxis or treatment of arteriosclerotic diseases, and a stent carrying the nanoparticles. Using the nanoparticles of the present invention, rupture of arteriosclerotic plaque can be suppressed, and they are useful for the prophylaxis or treatment of arteriosclerotic diseases.