An upper gastrointestina(l GI) series revealed a diverticulum in the anterior wall of the middle thoracic esophagus of a 72-year-old man. Endoscopy revealed a type 0-IIc lesion in the esophageal diverticulum. The margin of the lesion was unclear. Biopsy proved that it was squamous cell carcinoma. Endoscopic ultrasonography showed that the deepest layer of the tumor was the lamina propria mucosae (cT1a-LPM) and that the underlying muscularis propria was thinning. No distant metastasis or regional lymph node metastasis was detected. Diverticulectomy or endoscopic submucosal dissection (ESD) was out of indication due to the unclear margin and thin muscularis propria. We conducted mediastinoscopy-assisted esophagectomy. The pathological diagnosis of the resected specimen was moderately differentiated squamous cell carcinoma with invasion to the lamina propria mucosae (pT1a-LPM). Pathological examination proved the thinning of the underlying muscularis propria in the diverticulum. The patient is alive without recurrence at 6 months after surgery.