It has been clarified that self-efficacy (Bandura, 1977) predicts behavior and plays an important role in improving performance. However, there have been no reports of any practical trials for improving sports performance through the development of self-efficacy. The purpose of this study was to propose concrete application of the concept of self-efficacy to sports coaching, using base-running behavior in baseball as a theme. In Study 1, we examined the reliability (internal consistency and stability) of the Self-Efficacy Scale for Base-Running (SES-BR). In Study 2, we examined whether the intervention of the SES-BR could improve the self-efficacy and the performance of base-running as well as psychological competitive abilities such as “decisiveness,” “predictive ability,” and “judgment.” In Study 1, three coaches from a college baseball team extracted 31 types of base-running behavior that are considered critical in baseball games. In order to examine the reliability of the scale, the self-efficacy scores of the fielders in the team ( N =24) were measured twice: once on the first day of the season, and again, one week later. The results indicated Cronbach's a =.94 and a test-retest correlation coefficient r =.81 ( p <.001) for the SES-BR, thus verifying the scale's reliability. In Study 2, the college baseball players received cards on which the SES-BR was printed and were instructed to verify the 31 base-running behavioral items every five days. The results revealed a significant increase in the self-efficacy score during the intervention period ( p <.05). Moreover, the base-run errors—an index for base-running performance—decreased due to the intervention of the SES-BR: 8.30/game before the intervention, 6.77/game (-18%) in the first season, and 4.32/game in the second season (-48%). Furthermore, “decisiveness” and the comprehensive evaluation of psychological competitive ability, as evaluated by DIPCA (Tokunaga, 2001), showed significant improvement during the second intervention season ( p <.05 for both). Therefore, the intervention of the SES-BR, which was developed in this study and confirmed to be highly reliable, was suggested as a possible measure for improving self-efficacy and performance in base-running as well as “decisiveness” and overall psychological competitive ability.