Abstract: The present disclosure is to provide a method capable of producing an organic acid in high yield from an acetic acid strain using synthesis gas as a substrate. According to the present disclosure, the productivity of metabolites with C2 to C6 carbon atoms derived from synthesis gas and the selectivity of hexanoic acid production among metabolites can be improved through a first fermentation step of simultaneously providing a substrate comprising synthesis gas and sugar; and a second fermentation step of semi-mixotrophic fermentation of providing only the substrate comprising the synthesis gas. Therefore, the disclosure can contribute to sustainable chemical production using synthesis gas.