A 27-year-old man was referred to our hospital for a detailed examination of abdominal distention, bloody stool, anorectal pain, and weight loss. A colonoscopy revealed a circumferential type 2 tumor at 9 cm from the anal verge which was diagnosed as an adenocarcinoma based on biopsy. Contrast-enhanced CT of the abdomen showed an elevated perineal lipid concentration in the rectum(Ra)which was suspicious for clinical T4a stage, and simultaneous S7/8 liver metastasis. We strongly suspected familial adenomatous polyposis(FAP)because his mother had a past history of total proctocolectomy for FAP. We decided to first create a loop stoma at the transverse colon for the obstructive rectal cancer, and then administer neoadjuvant chemotherapy(mFOLFOX6 plus panitumumab). We performed total proctocolectomy with permanent stoma and S8 ventral resection for the liver metastasis after 5 courses of mFOLFOX6 plus panitumumab. As for clinicopathological findings, round 50 polyps were identified in the colon and rectum, and rectal cancer invaded into the muscularis propria. Finally, the patient was diagnosed as a clinically attenuated FAP with ypT2 rectal cancer.