An 81-year old man with a perirectal abscess was referred in May 2017 by another hospital. We observed swelling in the anal region at the 4 o'clock position and performed incisional drainage. Although this alleviated the pain and inflammation in the anal region, the irritation recurred in early June. The patient presented with bloody stools and a low-grade fever since late June. Pelvic magnetic resonance imaging(MRI)confirmed a solid tumor in the center of the lower rectum(Rb), outside of the anal fistula. We surmised this was rectal cancer. Colonoscopy revealed an ulcerative invasive(Grade 3)tumor extending more than halfway around the Rb; a biopsy confirmed a diagnosis of differentiated adenocarcinoma. Surgery was the preferred treatment option; however, as the patient also had the complication of anal fistula, there were concerns that the cancerous cells would contaminate the intraperitoneal area during surgery. We subsequently we decided to construct a colostomy and then start chemoradiotherapy. The patient began radiotherapy in the beginning of August, and received S-1 as a sensitizer. Contrast computed tomography(CT)and MRI at the completion of chemoradiotherapy confirmed that the rectal cancer had reduced in size. We scheduled later surgery, but the patient declined and preferred to continue with S-1. The tumor has continued to decrease in size, with good local control.