The present invention provides a fed-batch culture method comprising a step of fed-batch-feeding a carbon source base and a base in such a manner that the pH level can be maintained at a level suitable for the growth of microorganisms for fermentation of a carbon source. The present invention also provides a method for preparing organic acids using the fed-batch culture method. The present invention fed-batch-feeds a neutralizing agent such as ammonium bicarbonate, ammonium carbonate or alkali metal-containing weak base, and a carbon source substrate in preparing organic acids by microorganism fermentation. Thus, a pH level suitable for the survival of microorganisms for carbon source fermentation can be maintained, and the speed of injecting the carbon source base which is the source material can be appropriately adjusted. The present invention may improve productivity, yield rate and concentration of organic acids and may automatically inject a base and a carbon source substrate according to a variation in the pH level, thus improving reliability and convenience of a fermentation process operation.