An 80-year-old man was seen by his family doctor with chief complaints of fatigue and loss of appetite. Upper gastrointestinal endoscopy showed a type 2 tumor of the lower thoracic esophagus, and the patient was referred to our hospital. Squamous cell carcinoma was diagnosed on biopsy, and computed tomography (CT) showed multiple pulmonary metastases. The patient was diagnosed with Stage Ⅳ thoracic esophageal carcinoma, and was started on combined chemotherapy with 5-FU plus CDDP. Seven days after the start of chemotherapy, the patient developed mild intermittent pain. By day 18, the blood tests showed a marked inflammatory response, and a CT scan showed an abscess in the small bowel mesentery. We suspected an intra-abdominal abscess caused by small bowel perforation, and performed a partial resection of the small bowel and abscess drainage. Postoperatively, pathology tests revealed a diagnosis of a small intestinal diverticulum, which had penetrated the mesentery. We report our experience of a rare case of penetrating diverticulum of the small intestine that occurred during chemotherapy for esophageal cancer, and review the literature.