Inclusion Criteria:  Patients must provide written informed consent themselves to participate in this study Aged 20 or older at consent (male or female) Hospitalized due to worsening heart failure with both signs of congestion (such as edema, moist rales, and congestion on chest X-ray) and symptoms of heart failure (such as dyspnea on mild exertion or at rest) (any level of left ventricular ejection fraction) NYHA class II-IV Taking an ACE inhibitor or an ARB Can undergo randomization within 7 days of current hospitalization Patients who meet the following criteria of hemodynamic stability I. Systolic blood pressure ≥100 mm Hg II. No dose increase of intravenous diuretic within 6 hours before randomization III. No intravenous administration of vasodilator (such as carperitide or nitrates) or positive inotropic agent Patients who meet the following reference range for natriuretic peptide level from 48 hours before current hospitalization to the time of eligibility determination  NT-proBNP ≥1200 pg/mL or BNP ≥300 pg/mL  Exclusion Criteria:  Currently taking oral sacubitril valsartan or have taken it within 30 days prior to randomization History of hypersensitivity to ingredients in ARB, ACE inhibitor, or sacubitril valsartan; or expected to be contraindicated for or intolerant to any of these drugs History of angioedema Severe renal dysfunction (<eGFR 30 mL/min/1.73 m^2), on maintenance dialysis, or known bilateral renal artery stenosis (in patients with solitary kidney, known renal artery stenosis in the residual kidney) Severe liver dysfunction (Child-Pugh class C) Diabetic patients who are currently taking aliskiren fumarate Serum potassium ≥5.3 mEq/L or more Cardiogenic shock On cardiopulmonary support, with a left ventricular assist device, or on a ventilator Onset of stroke or acute coronary syndrome within 30 days prior to randomization History of surgical or percutaneous treatment of cardiovascular disease within 30 days prior to randomization Patients with an advanced plan for surgical or percutaneous treatment of cardiovascular disease or for coronary artery revascularization during an observation period Patients with an advanced plan for pacemaker implantation, cardiac resynchronization therapy, or electrical cardioversion during an observation period History or comorbidity of hypertrophic obstructive cardiomyopathy or infiltrative cardiomyopathy such as amyloidosis or sarcoidosis Active pericardial disease History of or awaiting heart transplant Severe chronic respiratory disease or active infectious disease Patients who are or might become pregnant or who are breastfeeding Patients whom a study investigator determined to be unsuitable for the study (such as patients with comorbid active malignancy)