A 61-year-old man with a 5-year history of anal fistula visited another hospital because of a swelling and a pain at the left side of the anus. The examination of the perianal region revealed a 5-cm diameter mass at 2 o'clock to the anus. The histological diagnosis of the lesion was well to moderately differentiated adenocarcinoma. A total colonoscopy confirmed the presence of a circular, ulcerated mass at the rectosigmoid junction, 20cm from the anal margin. A biopsy of the tumor showed a moderately differentiated adenocarcinoma. He was admitted to the hospital and underwent an abdominoperineal resection with a wide excision of the left perianal lesion. It is difficult to certify histologically that the carcinoma supervenes on anal fistula. In this case, the perineal tumor showed the same histological appearance as the rectal cancer and there was no mucoid discharge common in carcinoma of the anal gland origin. We considered that free tumor cells from the rectal cancer might have been implanted in the anal fistula.