Internal hernia after gastrectomy is a rare complication. It can progress rapidly to vascular disturbance, necrosis, and perforation, therefore early diagnosis and surgical treatment is essential. We present a case of internal hernia following laparoscopic-assisted proximal gastrectomy with jejunal interposition reconstruction in a 68-year-old man, who presented with acute abdominal pain and vomiting. Computed tomography showed a whirl sign, ascites, and a closed-loop formation of the small intestine. We diagnosed an internal hernia and performed emergency surgery. Laparotomy revealed chyle-like ascites and extensive small intestine with poor color. We recognized that about 20 cm of jejunum from the ligament of Treitz was strangulated behind the pedicle of the jejunum lifted during laparoscopic-assisted proximal gastrectomy. We relieved the strangulation, whereupon the color of the strangulated intestine was restored. Therefore, we did not perform intestinal resection and reconstruction. Finally, we fixed the jejunal pedicle and mesentery of the transverse colon. We report this case as there are few reported cases of internal hernia after laparoscopic-assisted proximal gastrectomy.