Ameloblastic carcinoma is a very rare malignant odontogenic tumor. We report a case of secondary-type ameloblastic carcinoma that extended transversally over almost half of the side of the face. Malignant transformation and identification of the malignant region in the large tumor were achieved using L-3-[F]fluoro-α-methyltyrosine (FAMT)-positron emission tomography (PET); FAMT is transported into cancer cells by L-type amino acid transporter 1 and shows high specificity for malignant tumors. The malignant region determined by microscopic evaluation of resected samples was similar to the region showing high FAMT uptake in PET. Using preoperative FAMT-PET and magnetic resonance imaging, we were able to achieve total resection of the very large tumor, while avoiding excessive resection that could cause severe functional loss or a poor aesthetic facial appearance. We used a modified Weber-Fergusson incision along the nasolabial fold to the labiajugal fold and reconstruction with a pectoralis major muscle flap, and this gave a good aesthetic outcome. Safe surgical resection was possible because preoperative three-dimensional computed tomography angiography was used to identify the position of the main trunk of the external carotid artery, which was closely aligned with the tumor in a posterior direction in the infratemporal fossa region.