Disclosed in this invention are a novel enzyme producing tartaric acid ether compound which catalyzes a reaction of producing a tartaric acid ether compound in which the phenolic hydroxyl group(s) of a flavonoid or its analogous compound having at least one phenolic hydroxyl group is (are) acted to the epoxy ring of an epoxysuccinic acid to form ether linkage of one or more tartaric acid residues, a process for producing a tartaric acid ether compound by acting the enzyme in the presence of an epoxy-succinic acid and a flavonoid or its analogous compound having at least one phenolic hydroxyl group, and novel tartaric acid ether compounds in which the epoxy ring of an epoxysuccinic acid is bound to the phenolic hydroxyl group(s) of a flavonoid or its analogous compound to form ether linkage of one or more tartaric acid residues. These compounds are useful as a component material of pharmaceuticals and functional foods.