A 65-year-old male visited our hospital because of fever and difficulty in walking. He was suffering from left-sided hypochondrial pain for a month. Laboratory tests performed on admission revealed a white blood cell count of 1700/μl and C-reactive protein level of 9.51mg/dl, which were suggestive of severe inflammation. Contrast-enhanced computed tomography revealed a subphrenic abscess around the spleen, which we considered to be caused by gastric penetration into the gastrosplenic ligament. Upper esophagogastroduodenoscopy revealed a gastric ulcer together with a fistula that connected to the left subphrenic abscess. We thus performed endoscopic transgastric drainage through the fistula. Antibiotics and a proton pump inhibitor were administered, and drainage was continued. The patient's clinical and inflammatory symptoms subsequently improved. We thus consider that endoscopic transgastric drainage is an appropriate treatment option for subphrenic abscesses.