The present invention provides an agent for treating and/or preventing adult T cell leukemia/lymphoma containing a compound having a specific chemical structure or a pharmaceutically acceptable salt thereof. Specifically, the present invention provides an agent for treating and/or preventing adult T cell leukemia/lymphoma containing, as an active ingredient, 7-chloro-2-[trans-4-(dimethylamino)cyclohexyl]-N-[(4,6-dimethyl-2-oxo-1,2-dihydropyridin-3-yl)methyl]-2,4-dimethyl-1,3-benzodioxole-5-carboxamide or a pharmaceutically acceptable salt thereof, or 7-bromo-2-[trans-4-(dimethylamino)cyclohexyl]-N-[(4,6-dimethyl-2-oxo-1,2-dihydropyridin-3-yl)methyl]-2,4-dimethyl-1,3-benzodioxole-5-carboxamide or a pharmaceutically acceptable salt thereof.
