A mutant microorganism of a rumen bacteria, the mutant microorganism having disrupted a lactate dehydrogenase gene (ldhA), a phosphotransacetylase gene (pta), and an acetate kinase gene (ackA) while maintaining a non-disrupted pyruvate formate-lyase gene (pfl),
- wherein the rumen bacteria has a property of producing succinic acid,
- wherein the rumen bacteria are included in a genus of which Mannheimia succiniciproducens is included, and
- wherein the amount of any other organic acid produced by the mutant rumen bacterium as a byproduct in the production of succinic acid by the mutant rumen bacterium is less than 1 wt % based on the weight of succinic acid that is produced.