Acute phlegmonous gastritis is a rare disorder in which bacterial infection occurs in the gastric wall. Gastrectomy involving the affected area has been thought to be an effective form of treatment. The authors report a case of a 32-year-old woman who had severe upper abdominal pain without signs of peritoneal irritation. Endoscopy showed edematous and reddened gastric mucosa with a mass lesion in the gastric antrum. Endoscopic ultrasonography showed thickening of the antral wall and a low-echoic mass in the gastric antrum, thought to represent a fluid collection. White pus was aspirated from the mass. Localized type of acute phlegmonous gastritis with a gastric abscess was diagnosed. Culture of the pus showed Streptococcus pneumoniae. Through early diagnosis without laparotomy, the patient's gastritis was successfully treated with antibiotics alone.