Infliximab is effective in the treatment of steroid-resistant Crohn disease. However, there is clinical concern about a possible correlation between an increased risk of anorectal cancer and infliximab treatment. We report a case of anorectal cancer in long-standing perianal Crohn disease. A 34-year-old patient with a long-standing perianal lesion of Crohn disease underwent 3 sessions of infliximab therapy. After therapy, the concentration of plasma CEA was 36.5ng/ml and rose to 91.4ng/ml. We suspected anorectal cancer, so abdominoperineal resection was performed. The histological findings indicated mucinous adenocarcinoma. Monitoring of patients with long-standing perianal Crohn disease is considered essential for early diagnosis of anal cancer after obtaining biopsy samples from perianal lesions. Additionally, when infliximab is started for perianal Crohn disease, thorough examination for perianal lesion should be performed.