A 53-year-old woman had had a pale-brown lesion on her right cheek and neck from birth. Since she was about 40 years old, part of the lesion had gradually elevated and increased in size. At the first examination, there was a huge mass measuring 110 x 70 x 35 mm on the right cheek. A wide local excision of the area including a superficial parotidectomy and submandibular lymph node dissection was performed, followed by full thickness skin grafting. Histological examination of the tumor revealed an invasive dermal neoplasm characterized by lobes that were composed of foamy cells stained with Sudan-IV mixed with many atypical cells showing remarkable variation in the shapes and sizes of their nuclei. The surrounding epithelial changes were consistent with nevus sebaceus. From these findings, the gigantic tumor was diagnosed as a sebaceous carcinoma arising in nevus sebaceus.