PURPOSE: To make it possible to determine vitamin C having a low concentration in high sensitivity and simply determine both of reducing type L-ascorbic acid and oxidation type dehydroascorbic acid.

CONSTITUTION: L-ascorbic acid or dehydroascorbic acid in a sample liquid is made to react with ascorbic acid oxidase and dithiothreitol to provide a cyclic reaction between L-ascorbic acid and dehydroascorbic acid. A reduced amount of dissolved oxygen in the sample liquid at this time is detected with an oxygen electrode and the concentrations of L-ascorbic acid and dehydroascorbic acid in the sample liquid are obtained from the resultant reduced amount of dissolved oxygen.