Primary adenocarcinoma arising in the sublingual gland is very rare. In this report, we have described the details of a case of adenocarcinoma of the sublingual gland. A 27-year-old Japanese male was referred to our department with a swelling of the floor of the mouth on the right side. The patient underwent a wide resection of the lesion and dissection of the right upper neck. Twelve months after his primary surgery, he was readmitted to hospital because of a metastasis in the lower lobe of the right lung and a right lower lobectomy was performed. He has undergone periodical controls for 3 years. No sign of recurrence or metastasis has been observed.