The present invention relates to a composition for preventing or treating cancers, containing a proteasome inhibitor and a dihydropyridine-based compound as active ingredients. The proteasome inhibitor and the dihydropyridine-based compound, according to the present invention, do not induce cancer cell death when used as individual treatments, but when combined in treatment, cancer cell death is effectively induced by the synergy thereof. That is, the dihydropyridine-based compound effectively enhances the anti-cancer effect of the proteasome inhibitor, and thus induces cancer cell death, thereby exhibiting a very excellent anti-cancer effect. Therefore, the proteasome inhibitor and the dihydropyridine-based compound according to the present invention can be utilized for the prevention, alleviation, or treatment of cancer, and the dihydropyridine-based compound can be utilized as an anti-cancer adjuvant.