Eosinophilic enteritis is an uncommon disease of unknown cause characterized by eosinophilic infiltration in various areas of the gastrointestinal tract with symptoms. It is generally classified according to the layer of the gastrointestinal tract involved. Eosinophilic infiltration of the serosa is the rarest form of presentation and may manifest eosinophilic ascites. We report a case of a 47-year-old man who experienced progressing abdominal pain. A diffuse erythematous change of the gastric mucosa was observed on gastrofibroscopy. An abdominal computed tomography and colonoscopy showed diffuse wall thickening of the small bowel and colon with a small amount of ascites. Eosinophilic infiltration was confirmed by multiple biopsies of the gastrointestinal tract and peritoneal fluid analysis. The patient was treated with corticosteroid and responded dramatically.