Granulocyte colony-stimulating factor (G-CSF)-producing thymic carcinoma is extremely rare. A-66-year-old man presented with an anterior mediastinal mass, and underwent surgical biopsy. He had marked leukocytosis, and his serum levels of G-CSF and interleukin-6 were elevated. Histologically, the tumor consisted of squamous cell carcinoma, which showed positive immunoreactivity for G-CSF. He was treated with thoracic radiotherapy, and chest imaging revealed a marked reduction of tumor size. He was doing well at 8 months after tumor diagnosis.