An 80s male, with a medical history of hypertension, hyperuricemia, and atrial fibrillation, visited our emergency outpatient department with vomiting and diarrhea as the chief complaint in August 2017. The blood examination revealed a high level of inflammatory reaction. The plain abdominal CT revealed fluid retention contacting the small intestine and intraabdominal free gas. We diagnosed the case as a small intestinal perforation, following which we performed emergency surgery. The small intestine was perforated, and an abscess cavity was formed between the transverse mesocolon and mesentery proper. Thus, the abscess was removed, and about 30 cm of the small intestine, including the perforated site, was resected, followed by the reconstruction. The resected specimens revealed squamous cell carcinoma at the small intestinal perforated site. Lung squamous cell carcinoma was diagnosed by subsequent chest CT and immunostaining. We administered 3 courses of chemotherapy combined with carboplatin and albumin-bound paclitaxel. Although the effect was partially observed, interstitial pneumonia occurred, which was inferred to be drug-induced. The patient died in 195 days following the surgery. Herein, we reported a case of lung cancer, which was diagnosed on the detection of gastrointestinal perforation caused by a small intestinal metastasis.