Surfactin is an anionic surfactant generated by bacteria. Although it has high ability to decrease interfacial tension (IFT) between oil and water, it binds with bivalent cations and forms precipitation. Because the precipitation causes the significant reduction of reservoir permeability, surfactin cannot be applied to EOR in oil reservoir whose bivalent cations concentration is more than 100 ppm. This study investigated methods for applying surfactin to reservoir containing bivalent cations with high concentration. Screening of an effective binding inhibitor was carried out by measuring turbidity of the solution containing 0.3 wt% of surfactin, 900 ppm of calcium ion, and inhibitor candidates such as alcohols, chelating agents, cationic surfactants, and ion capturing substances. Influence of the inhibitors on surfactin capacity for decreasing IFT was also evaluated by measuring IFT between the solution and oil. The best inhibitor was finally selected through the injectivity tests using Berea sandstone core which was saturated with calcium solution. EOR potential of the solution containing the inhibitor was evaluated by the core flooding experiments. Citric acid and trisodium citrate inhibited binding of surfactin with calcium ion with lower concentration such as 0.6 wt%, they were selected as potential inhibitors and subjected to the IFT measurements. Both of them had strong potential as co-surfactants of the surfactin because IFT was greatly decreased to less than 0.1 mN/m which was less than a tenth as compared with IFT between the pure surfactin solution and oil. Trisodium citrate however caused significant permeability reduction on the injectivity tests whereas citric acid could be injected into the core without permeability reduction. The high pH value of trisodium citrate solution might cause the dissolution of ferrum and aluminum in the core and the colloids of ferrous hydroxide and aluminum hydroxide were formed in the core, which brought the significant permeability reduction. Citric acid was selected as the best inhibitor and subjected to the core flooding experiments. 25 % of oil remaining after primary recovery was recovered by injecting the solution containing 0.3 wt% of surfactin, 0.6 wt% of citric acid and 900 ppm of calcium ion. Rise in the differential pressure was not found during the injection of the solution, which suggested that citric acid was effective for inhibiting the precipitation in oil reservoir. Moreover, 25 % of recovery factor was 5 % higher than the recovery factor obtained by injecting pure surfactin solution. Citric acid is also effective for enhancing the surfactin capacity for increasing the recovery factor. Citric acid has dual role as the binding inhibitor and co-surfactant. Because citric acid is environmentally friendly and cheap chemical, it can be promising additive which increase the applicable reservoir and potential of surfactant EOR.