A 54-year-old man was referred to our hospital because of a huge, unresectable rectal cancer occupying his entire pelvic space with a solitary liver metastasis. He had undergone a laparotomy for surgical resection, but ended up with a sigmoid colostomy due to possible invasion into the urinary bladder and pelvic wall. At the completion of seven cycles of FOLFOX regimen, radiographic examination revealed remarkable reduction of the primary rectal tumor and regional lymph nodes, and also a complete response (CR) of the liver metastasis. The tumor was extirpated without any macroscopic residues by a low anterior resection of the rectum, along with a partial resection of the urinary bladder and seminal vesicles. Since pathological and immunohistochemical examinations showed no viable cancer cells in any parts of the resected specimens, the lesion was regarded as a pathologically CR. Analysis for single-nucleotide polymorphisms in the genes involved in nucleotide excision repair, excision repair cross-complementing group 1 and xeroderma pigmentosum group D, showed a genotypic pattern sensitive to oxaliplatin. To our knowledge, this is a rare case of an initially unresectable primary rectal cancer, which was down-staged to a pathologically CR by FOLFOX chemotherapy instead of chemoradiotherapy.