This invention relates to a process for producing lactic acid or its salts that can be performed easily, reduce complicated steps, and provide high lactic acid yield and high productivity, wherein said process comprising the following steps: (a) cultivating thermotolerance Bacillus genus bacteria to obtain a seed culture; (b) increasing cell number of bacteria by inoculating the seed culture obtained from step (a) into a fermenter containing an initial carbon source under an aerobic condition; (c) fermenting the seed culture obtained from step (b) in the fermenter under a microaerobic condition to obtain lactic acid or its salts; wherein the step (b) comprising at least one addition of the carbon source under any one of the following conditions, which are independent to each other, to increase a concentration of the carbon source: - when the concentration of carbon source in the fermenter reduces to 50 % or less comparing to the initial concentration; - when the step (b) is carried out for a time of at least
