Despite established implantable cardioverter-defibrillator (ICD) therapy and catheter ablation for sustained ventricular tachycardia (VT) in patients with ischemic heart disease (IHD) and reduced left ventricular ejection fraction (LVEF), the efficacy of catheter ablation in patients with nonsustained VT has been not yet clarified. The incidence of appropriate ICD therapy itself has been reported to be a worse prognostic factor in patients with reduced LVEF. Therefore theoretically the inhibition of these ventricular incidences can result in the prognostic improvement.To suppress ventricular arrhythmias aside from antiarrhythmic agents, catheter ablation has been developed prominently in this decade along with the technological improvement such as irrigated ablation catheters, three-dimensional mapping systems, multi-polar catheters, and image integration system with CT and MRI. The rationale of this trial is to study the efficacy of the eradication of arrhythmogenic substrate in ischemic cardiomyopathy with reduced LVEF and nonsustained VT on prevention of the occurrence of sustained VT/VF and ICD therapies.