A 68-year-old man diagnosed with type 0-Ⅰgastric cancer by gastrointestinal endoscopy underwent urgent distal gastrectomy due to a perforation during endoscopic submucosal resection. Pathological examination revealed pT3N2M0, pStage ⅢA. TS-1 was administered as adjuvant chemotherapy. Laboratory examinations 10 months after surgery revealed leukocytosis (19,100/mL). Positron emission tomography-CT demonstrated metastases in the bone marrow and ascending colon as well as around the liver. Chemotherapy using nab-PTX had poor efficacy and the leukocytosis worsened. Serum granulocyte- colony stimulating facto (r G-CSF) was high at 1,640 pg/mL, and immunohistochemical staining was positive for G-CSF. Thus, the patient was diagnosed with G-CSF-producing gastric cancer. The tumor was also positive for HER2 antibody by immunohistochemical staining. Combination therapy using TS-1 plus CDDP plus trastuzumab resulted in a good response, and the leukocytosis and elevated serum G-CSF gradually improved. The patient is living 30 months postoperatively and remains on chemotherapy.