A novel microorganism capable of producing organic acids, Mannheimia sp. 55E, and a process for producing organic acid through anaerobic and aerobic incubation using the novel microorganism are provided. The method of producing an organic acid using the microorganism involves incubating Mannheimia sp. 55E with Accession Number KCTC 0769BP in a medium under anaerobic or aerobic conditions and obtaining an organic acid from the medium. Mannheimia sp. 55E produces succinic acid, lactic acid, and formic acid under anaerobic conditions saturated with CO.sub.2, and succinic acid, lactic acid, acetic acid, and formic acid under aerobic or N.sub.2 anaerobic conditions. Mannheimia sp. 55E is a facultative anaerobe tolerant of oxygen. Thus, the use of Mannheimia sp. 55E in producing organic acids can eliminate a problem of process instability, which would occur by the presence of oxygen in a fermentation process of producing organic acids using an obligate anaerobic microorganism.