A 41-year-old woman arrived at our emergency department with semicomatous mentality due to aborted cardiac arrest from ventricular fibrillation, which occurred during resting state. Prolonged QT interval in condition of hypokalemia and hypomagnesemia was observed on initial electrocardiography. However, even after normalization of serum potassium, magnesium and mentality, QT prolongation was sustained. In addition, genetic analysis showed KCNH2 mutation which was consistent with type 2 congenital long QT syndrome (LQTS). In this case, the patient was treated with an implantable cardioverter defibrillator and genotype specific medical therapy with potassium channel opener.