A 6-month-old male infant was referred to our hospital with diagnosis of severe left heart failure stemming from severe mitral valve regurgitation (MR). He showed severe decompensation, and emergency operation was performed. Intraoperative transesophageal echocardiogram showed ruptured chordae of the posterior leaflet at P1 and P3 sites. The both sites of the posterior leaflet were resected and sutured. Bilateral Kay annuloplasty and partial annuloplasty at the sites of P1 and P3 were also performed. Postoperative echocardiogram showed only trivial MR. He made successful recovery after surgery.