We report on a patient with a prior history of squamous cell carcinoma (SCC) of the oral mucosa, who developed a second primary SCC on the cutaneous surface of the flap 16 years after oral reconstruction. A 75-year-old man underwent surgical resection of buccal mucosal SCC and reconstructive surgery with a forearm flap in 1995. Furthermore, he underwent the removal of a tongue SCC adjacent to the skin flap in 2007. In 2011, 16 years after the first operation, an SCC was found that had developed in the skin flap of the free forearm flap. It was resected and the area of resection was reconstructed with a pedicled pectoralis major myocutaneous flap and delto-pectoralis skin flap. After pathological re-examination of the tongue SCC in 2007, it was identified as a second primary SCC in the skin flap. As the carcinogenic mechanism in this case, we speculated that: at first, mucosal epithelial cells with genetic alteration adjacent to the skin flap grew in a patch of the normal skin epithelium and, through clonal expanstion, ingrew in a lateral direction, eventually developing a field lesion, taking over the normal skin epithelium; second, through prolonged exposure to factors such as saliva or dietary substances, neoplastic changes were triggered in the skin. We should follow patients closely for the long-term after reconstructive surgery due to the prospect of squamous cell carcinoma arising in the skin of a free flap.