Systolic anterior motion (SAM) is a common complication of mitral valve repair surgery and occasionally requires further treatment. A 56-year-old woman with severe mitral regurgitation accompanied by posterior leaflet prolapse underwent mitral valve plasty including hour-glass-shaped resection, chordal replacement, and interrupted commissural band annuloplasty. The mitral valve was exposed via a right-sided left atriotomy. We found a large thick P2-3 scallop (27 mm in height) with ruptured and elongated chordae. After repair, transesophageal echocardiography (TEE) revealed SAM of the anterior mitral leaflet and severe mitral regurgitation upon weaning from the cardiopulmonary bypass. Although catecholamine was discontinued and volume loading applied, the SAM did not improve. We decided to revise the mitral plasty. Therefore, although the height of the P3 scallop after resection remained 17 mm, neochordae were placed once more on the basal side of the posterior leaflet, and the leaflet was shortened by placing a continuous suture near the annulus. This reduced the height of the posterior leaflet and moved the co-aptation line posteriorly. After this repair, TEE showed that the SAM had disappeared. Thus, repositioning the neochordae and shortening the posterior leaflet by applying a continuous suture effectively and rapidly eliminated the problem.