A 62-year-old woman received chemotherapy for breast cancer with bone metastasis and malignant pleural and pericardial effusion. She was examined by imaging for progressive cognitive impairment and headache. Enhanced MRI findings showed multiple solid tumors on brain surface, and brain perfusion scintigraphy showed blood flow decrease in both parietal lobes. She was diagnosed with secondary dementia due to leptomeningeal metastases of breast cancer, and whole brain external irradiation was performed(30 Gy/15 Fr). After treatment, multiple tumors were decreased in size and her cognitive impair- ment was improved.