We present a patient with acute phlegmonous gastritis after a gastric operation. The patient underwent subtotal gastrectomy and lymph node dissection following the diagnosis of early gastric cancer. The postoperative clinical course was good, but on the 14th postoperative day, he had a sudden onset of upper abdominal pain and abdominal muscle guarding. At the same time he had high fever and complete anorexia. Laboratory studies showed marked leukocytosis (28,000/mm3) with a shift to the left, and computed tomography showed marked thickness of the anterior wall of the remnant stomach. Furthermore, a discharge of approximately 500 ml of a yellowish turbid fluid through a nasogastric tube was observed. Because the patient had no evidence of anastomotic leakage or intra-abdominal abscess, the diagnosis was very difficult. But eventually, he was diagnosed to have acute phlegmonous gastritis of the remnant stomach by the findings of CT and the pus discharge of a nasogastric aspirate. Acute phlegmonous gastritis is a rare inflammatory disorder of the stomach, and it involves high mortality because it is so often misdiagnosed and treatment is initiated too late. The present case is the first surgically related case of acute phlegmonous gastritis after a gastric operation.