A 62-year-old Japanese man presented with a 1-month history of inter-digestive epigastralgia. His family history included a sister with gastric cancer. Gastroendoscopy and gastrography demonstrated a type-2 tumor in the upper region of the stomach. CT scan and fluorodeoxyglucose-positron emission tomography (FDG-PET) scan demonstrated gastric cancer and its metastatic lymph nodes. The patient underwent total gastrectomy with splenectomy and extended lymph node dissection. Although postoperative adjuvant chemotherapy by S-1 was started, the deteriorating condition of the patient prevented drug administration and even eating meals. On the 19th postoperative day (POD), FDG-PET scan of the body demonstrated new uptake in the liver and lymph node around the aorta. Without any sign of infection, leukocytosis developed around the 30th POD. On the 49th POD, remarkable uptake in the whole upper abdomen was detected on FDG-PET scan. Finally, leukocyte count increased to 125,200 and granulocyte colony stimulating factor (G-CSF) was elevated to 28 pg/ml on the 54th POD. The patient died of multiple liver metastases and carcinomatous peritonitis only 56 days after surgery. G-CSF-producing tumor is a rare but aggressive disease, particularly as recurrent tumor. If leukocytosis is detected in relation to a non-lympho hematopoietic malignant tumor, G-CSF-producing tumor should be considered and FDG-PET scan is recommended for early detection. Chemotherapy for G-CSF-producing tumor must be conducted as soon as possible.