An antitumor effect enhancer for enhancing the antitumor effects of an antitumor substance, which comprises a compound represented by the following formula (1): wherein each R independently represents a hydrogen atom, a lower alkyl group, an aryl group, an allyl group, an aralkyl group, a halogen atom, a hydroxyl group, an amino group, R′O—, R′(C═O)—, R′(C═O)O—, or R′O(C═O)—, wherein R′ is an alkyl group containing 1 to 5 carbon atoms, or a pharmaceutically acceptable salt thereof.