An antitumor effect enhancer for enhancing the antitumor effects of an
antitumor substance, which comprises a compound represented by the
following formula (1): ##STR00001## 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.dbd.O)--, R'(C.dbd.O)O--, or R'O(C.dbd.O)--, wherein
R' is an alkyl group containing 1 to 5 carbon atoms, or a
pharmaceutically acceptable salt thereof.