A method for producing a basic acid such as L-lysine is provided. A basic amino acid or a fermentation product containing the same is produced by a method including the following steps (A) and (B): (A) a step of culturing a microorganism able to produce a basic amino acid in a culture medium so that bicarbonate ions and/or carbonate ions serve as counter ions for the basic amino acid, to obtain a fermentation broth containing the basic amino acid; (B) a step of subjecting the fermentation broth to a heat treatment under a pressure sufficient for preventing generation of carbon dioxide gas from the fermentation broth.