Organic acids are abundant biogenic organic molecules in the earth's surface environments. To evaluate the effect of organic acids on the precipitation rate and polymorphism of calcium carbonate (CaCO_3) minerals, we conducted precipitation experiments of CaCO_3 minerals by the batch method using 100 ml solutions containing citric acid, malic acid, and acetic acid with Mg^<2+> ions at 25℃ for 10 days. Each solution contained 5.0 mM Ca^<2+> and Mg^<2+> in addition to 20.0 mM HCO_3^- and 0.01, 0.05, 0.1, 0.5, and 1.0 mM organic acid. In addition, adsorption experiments with these organic acids were conducted on the surfaces of calcite and aragonite to reveal the adsorption affinity of organic acids for CaCO_3 minerals. Results confirmed that citric acid significantly decreased the precipitation rate of CaCO_3 minerals and strongly favored the formation of calcite as the predominant polymorph by increasing its concentrations. Moreover, malic acid decreased the precipitation rate and favored the formation of calcite to a lesser extent than with citric acid. Acetic acid showed no significant effect on the precipitation rate and polymorphism of CaCO_3 minerals. Adsorption experiments revealed that adsorption affinity of organic acids was much higher for the surface of aragonite than that for the calcite surface, and showed adsorption amounts in the following order: citric acid > malic acid > acetic acid. Thus, the effect of organic acids on the precipitation rate and polymorphism of CaCO_3 minerals were fundamentally caused by the adsorption of organic acids on the CaCO_3 surface. In this process, higher adsorption affinity of the aragonite surface with organic acids led to a greater adsorption, resulting in greater inhibition of aragonite precipitation and favorable formation of calcite as the dominant polymorph.