A 55-year-old woman had received total gastrectomy for advanced gastric cancer in March 2002, and was subsequently treated with adjuvant chemotherapy using oral anti-metabolite TS-1 for 21 months. She was well with no evidence of recurrence of gastric cancer, but leukocytosis was found in June 2005. The analysis of bone marrow revealed that Philadelphia (Ph) chromosome and bcr-abl fusion gene were positive. On the basis of these findings, the chronic phase of secondary chronic myeloid leukemia (CML) was diagnosed. Three months after being started on imatinib therapy, Ph chromosome positive cells disappeared in the bone marrow, and a complete cytogenetic response was achieved. Although CML is rare in secondary leukemia, this is, to our knowledge, the first reported case of therapy-related CML following TS-1 treatment. The present case suggested that imatinib therapy was also effective for secondary CML as well as de novo CML.