The association of diabetes mellitus and insulinoma is unusual. We are reporting the case of a 58 years old patient having diabetes mellitus type 2 for several years. This patient was well balanced with oral anti-diabetic treatment. However, the diagnosis of insulinoma was discussed due to recent episodes of hypoglycemia that persisted even after stopping the treatment. Abdominal CT allowed the topographic diagnosis. The patient underwent a caudal pancreatectomy. Furthermore, the postoperative period shows that the diabetes mellitus requires the oral anti-diabetic treatment and basal insulin have to be stable. Then, the occurrence of hypoglycemia in the diabetic mellitus type 2 and, especially the persistence after discontinuation of therapy, suggest the unusual diagnosis of insulinoma as illustrated in our observation.