Although S-1 is an effective oral anticancer drug in patients with metastatic breast cancer, it is difficult for some patients to continue taking S-1 because of its side effects in the approved regimen of 4 weeks of administration followed by a 2-week rest. When S-1 is administered for 5 days followed by a 2-day rest(5-day on/2-day off), the drug concentration is almost equal to that of the approved regimen, and it can be administered for longer without deterioration of its clinical effect. We retrospectively analyzed the effect and safety in 25 cases in which S-1 was administered using the "5-day on/2-day off" regimen in patients with metastatic breast cancer between November 2006 and August 2014 in our hospital. Patients were all female, and their median age was 68 years(44-87). ER was positive/negative in 15/10 cases, and PS 0/1/2were found in 8/ 10/7 cases. They had no prior chemotherapy and had measurable lesions. S-1 was administered at a dose of 80mg/m2 twice a day on a "5-day on/2-day off" schedule and was reduced when its side effects were appeared. The median treatment duration was 25(3-214)weeks, and CR/PR/long SD/SD/PD as clinical responses were observed in 0/8/5/5/7 cases. The overall response rate was 32% and clinical benefit rate was 52%. There was no difference in response rates whether visceral metastases were present or not. In terms of hematologic toxicity, anemia was seen in one case, and there were no cases of neutropenia. In non-hematologic toxicity, no more than Grade 3 side effects were shown. Discontinuance was observed in only one case because of diarrhea. A "5-day on/2-day off" regimen of S-1 in metastatic breast cancer is well tolerated, and we can continue administering it to elderly patients or those with poor PS without reducing QOL; thus, it can be considered as one of the effective metronomic treatments. In the future, a prospective study is warranted to ascertain these results.