A 52-year-old Japanese woman was diagnosed as having advanced BC with a sensitive estrogen receptor and without human epidermal growth factor receptor 2. This diagnosis took place about 3 years prior to the admission described in the present report. She received four courses of combination therapy with epirubicin, cyclophosphamide, and fluorouracil (ECF) for 6 months, and then eight courses of docetaxel (DTX) for 10 months. After these two regimens, a malignant pleural effusion developed; therefore, the disease was considered progressive. After DTX treatment, the patient received two estrogen receptor inhibitors, tamoxifen for 6 months, fulvestrant for 4 months, and an aromatase inhibitor (letrozole) for 11 months. Despite this, the patient’s BC metastasized to the liver; the attending physician therefore considered the disease progressive and began treatment with eribulin. Five days after she began her second treatment with eribulin, she was admitted to our hospital for dyspnea. Upon admission, her initial vital signs included a body temperature of 37.5 °C and an oxygen saturation of 88% in room air. A physical examination revealed fine crackles in both lower lung fields. Her laboratory test results were as follows: white blood cells—1200 cells/µL (neutrophils—600 cells/µL); serum lactate dehydrogenase (LDH)—749 IU/L (normal: 119–229 IU/L); serum sialylated carbohydrate antigen Krebs von den Lungen-6 (KL-6) level—3782 U/mL (normal: < 500 U/mL); serum surfactant protein-D (SP-D)—158 ng/mL (normal: < 110 ng/mL); serum C-reactive protein (CRP)—3.9 mg/dL (normal: < 0.3 mg/dL); and plasma (1-3)-beta-d-glucan—12 pg/mL (normal: < 20 pg/mL). The patient was negative for serum antibodies associated with connective tissue diseases. Furthermore, her sputum, urine, and blood cultures yielded no microbial growth, and she was negative for a rapid flu test, cytomegalovirus antigen, Mycoplasma antigen in the pharynx, and Streptococcus pneumoniae and Legionella antigen in the urine. A chest radiograph showed consolidation in the right lower lung field, and a chest high-resolution computed tomography (HRCT) scan showed consolidation with air bronchograms along the bronchovascular bundles in both lower lobes. Prior to the admission described in the present study, the patient had no pre-existing interstitial shadow on her chest radiograph and HRCT. Moreover, she showed no evidence of heart or renal failure; that is, her brain natriuretic peptide and creatinine levels were within the normal range. A year before the admission described in the present report, the patient was prescribed brotizolam, tramadol, loxoprofen, and famotidine to treat insomnia, chronic gastritis, and carcinomatous pain. Upon admission, we treated her with the antibiotic cefepime for 5 days, as well as with carbocysteine and dextromethorphan hydrobromide hydrate for sputum and cough. However, her respiratory symptoms and radiological findings were not improved by this treatment. Five days after her admission, we performed a bronchoalveolar lavage (BAL) in the right B4 bronchus. The total cell count, lymphocyte, neutrophil, eosinophil, and macrophage percentages, and CD4/CD8 ratio values in the BAL fluid (BALF) were 10.8 × 105 cells/mL (normal range: 0.7–2.0 × 105 cells/mL), 13, 2, 0, 85%, and 0.63 (normal range: 1.5–3.2), respectively. The polymerase chain reaction (PCR) assay of the BALF for Pneumocystis jirovecii was negative. Based on these findings, including the clinical course and chest HRCT, we concluded that the patient had developed IP, specifically organized pneumonia (OP), and we therefore initiated treatment with prednisolone (1 mg/kg/day), gradually decreasing the dosage every 2 weeks. The patient’s respiratory condition and chest radiograph findings had improved after 2 weeks of prednisolone treatment.