Walled-off necrosis (WON) is a pancreatic fluid collection, which contains necrotic tissue after four weeks of the onset of acute pancreatitis. Interventions are required to manage patients with infected WON, for which endoscopic ultrasonography (EUS)-guided drainage has become a first-line treatment modality. For patients who are refractory to EUS-guided drainage, the step-up treatment including endoscopic necrosectomy (EN) and/or additional drainage is considered to subside the infection. Recent evidence suggests that EN immediately after EUS-guided drainage may shorten treatment duration without increasing adverse events. In this randomized trial, the investigators will compare treatment duration between EN immediately after EUS-guided drainage versus the step-up approach in patients with symptomatic WON.