A process for producing a high purity 3,5-dihydroxy6-heptenoic acid
derivative by controlling the content of impurities such as denatured
substances, is provided. When a 3,5-dihydroxy-6-heptenoic acid derivative
is produced by a process which comprises a step of contacting the
3,5-dihydroxy-6-heptenoic acid derivative of the formula (1) wherein R is
a C.sub.1-4 alkyl group, with a C.sub.1-4 lower alcohol-containing
solvent, an alcohol containing solvent having its content of an oxidizing
substance lowered, is used to at most 0.05 molar equivalent to a
3,5-dihydroxy-6-heptenoic acid derivative, to suppress impurities
contained in the 3,5-dihydroxy-6heptenoic acid derivative. ##STR00001##