Cold snare polypectomy (CSP) should be performed for benign lesions, though an accurate diagnosis is sometimes difficult with only white light observation. Irregular findings by narrow-band imaging (NBI) are useful for differentiating malignant lesions from benign lesions, and cases with this finding are not expected for CSP. We present a diminutive T1 cancer resected by CSP as a reflection case. A 68-year-old man underwent colonoscopy for surveillance after polypectomy. A reddish polyp 4 mm in size was detected at the rectum. White light observation showed no depression, but a slight, heterogeneous color change. NBI magnification showed irregular vessel and surface patterns. The polyp was diagnosed as intramucosal cancer. Even though cancerous lesions are regularly resected by endoscopic mucosal resection (EMR), this polyp was resected by CSP in daycare surgery because the patient requested not to be treated by EMR but by CSP, which needed an admission to our institution. The surgeon thought the polyp could be completely resected by CSP. It was thoroughly resected, and a histological examination showed submucosal cancer with a positive vertical margin. Additional surgical resection was not accepted by the patient, since he had received total gastrectomy for gastric cancer and a right hemicolectomy for colonic cancer in the past 7 years. He underwent follow-up colonoscopy 2 months after the CSP. Although there were no recurrent endoscopic findings, endoscopic submucosal dissection was performed to the scar area. The histological examination showed no residual tumor. In conclusion, CSP should only be adopted for benign cases, as cancerous lesions have a possibility for invading the submucosa, like in our case.