Abstract: The present invention relates to a composition comprising, as active ingredients: (1) a biguanide-based compound or a pharmaceutically acceptable salt thereof; (2) 2-deoxy-D-glucose; and (3) inositol hexaphosphate or a pharmaceutically acceptable salt thereof, inositol, or a mixture thereof. The composition according to the present invention exhibits a synergistic anticancer effect by appropriately combining specific drugs having a problem that needs to be used in a large amount, thereby making it possible to kill cancer cells in a small amount and effectively treat the cancer. Furthermore, the composition of the present invention may kill only cancer cells without side effects by exhibiting a specific toxic effect on cancer cells without showing toxicity on normal cells and thus be usefully used as an anticancer agent and for preventing or improving cancer.