We describe a sublingual type of dermoid cyst that caused obstructive sleep apnea syndrome (OSAS). A 20-year-old man was referred to our clinic because of a swelling of the submental region, insomnia, and daytime sleepiness. The clinical diagnosis was a dermoid cyst or an epdermoid cyst accompanied by OSAS. The occurrence of breathing disorders and daytime drowsiness were monitored to evaluate OSAS before and after surgery. The preoperative Apnea-Hypopnea Index (AHI) and Epworth Sleepiness Scale (ESS) were 39 and 14, respectively. The postoperative AHI and ESS were 10 and 1, respectively. After surgery, insomnia and daytime sleepiness improved. As of 3 years after the operation, there is no evidence of recurrence.