Sebaceous carcinoma is a rare primary neoplasm of the lacrimal gland and, to the best of our knowledge, only six cases have been reported previously. Sebaceous carcinoma of the orbit more commonly occurs as secondary invasion from the eyelid, but may occur by way of metastatic spread from elsewhere in the body. We describe a patient who presented with a rapidly growing neoplasm of the lacrimal gland which, histologically, was a sebaceous carcinoma. The eyelid was entirely normal on examination. In this patient we found a single tumor cell line within the normal lacrimal gland. In this article we describe the diagnosis and treatment with carbon ion radiotherapy of primary sebaceous carcinoma of the lacrimal gland, a condition not previously reported in Japan.