Provided is a method for producing an organic acid, whereby it becomes possible to directly recycle carbon dioxide and a light energy from a microalga to produce an organic acid effectively. The method involves the step of culturing a microalga under temperature conditions of 35 to 40篊, wherein the microalga is further cultured in a system containing carbonate ions and/or bicarbonate ions. In this manner, an organic acid can be produced effectively through the photosynthesis of the microalga and by the action of a carbon source incorporated into the microalga. Among organic acids, succinic acid, for example, has been produced using petroleum or the like as a raw material. In contrast, according to the method of the present invention, an organic acid can be produced by effectively utilizing a biomass. The produced organic acid can be used effectively in foods, medicines and other various fields.