To expand the utilization of violet sweet potato by extracting a violet pigment extract from the raw violet sweet potato with an acidic solution, for example, an organic acid and using the extract for vinegar.

The pigment is extracted from raw violet sweet potato with at least one selected from organic acids and mineral acids, preferably at least one 0.5-2 wt.% of citric acid and vinegar and the resultant pigment solution is used for producing vinegar. In a preferred embodiment, the residue after pigment extraction with an acidic solution is processed to a paste and it is served as a food.