A 78-year-old woman was examined in the outpatient department with a chief complaint of swelling of the left breast. Examination confirmed a 10 cm mass in the left breast as along with edema and redness of the skin, following which a diagnosis of invasive micropapillary carcinoma was made after biopsy. The CT imaging showed left chest wall invasion, multiple axillary lymph node metastases, and left carcinomatous pleuritis. Since this a case of advanced breast cancer, we initiated treatment with bevacizumab plus paclitaxel. After 8 months, her medication was changed to eribulin, owing to progression of the cancer, which continued even up to 4 months. We then initiated abemaciclib plus letrozole therapy as the third treatment. We observed tumor reduction and clearing of pleural effusion with no serious adverse events, and continued her therapy for 11 months before the cancer progressed. We report a case of chemotherapy-resistant breast cancer and carcinomatous pleuritis in an older adult patient for which abemaciclib plus letrozole therapy was effective.