Since the Inoue balloon was first introduced for percutaneous mitral valvuloplasty (PMV) in 1984, this procedure has come into widespread use because of its effectiveness, simplicity, and reduced exposure to X-ray radiation. It's the procedure's complications include cardiac tamponade, atrial septal defect, thromboembolism, ventricular perforation, mitral regurgitation, and rarely balloon rupture. We report a case of Inoue balloon deformity during PMV in 62-year old woman with rheumatic mitral stenosis. Echocardiography revealed severe rheumatic mitral stenosis with a valvular area of 0.95 cm² (by pressure half-time method), and an Echo score of 10 points. The PMV with Inoue balloon 28 mm was performed. We inflated the balloon to 28 mm in diameter first, and to 29 mm second. A bulging deformity with asymmetrical overinflation of one side of both proximal and distal balloon was recognized. A bulging deformity at the proximal part of Inoue balloon after second inflation. Balloon was not ruptured. Following completion of the procedure, the mitral valve area increased to 1.8 cm². Moderate mitral regurgitation (grade II) was newly developed. This may be the first case of asymmetrical one side inflation and focal bulging deformity reported in Korea.