Metaplastic carcinoma is a rare histological malignancy, often triple-negative, and has a poor prognosis. Here, we report a case of breast cancer in which the primary lesion degenerated into squamous cell carcinoma(triple negative)after drug treatment for invasive ductal carcinoma(Luminal type). The patient was a 41-year-old woman who was diagnosed with Stage Ⅳ left breast cancer T2N2bM1(HEP)(ER 90%, PR 70%, HER2 2+, FISH-)at another hospital and participated in the PATHWAY study(tamoxifen plus goserelin plus palbociclib/placebo). Since the primary lesion and liver metastasis increased in size, the study was discontinued after 8 weeks. She was treated at our hospital thereafter, with capecitabine plus cyclophosphamide, palbociclib plus fulvestrant plus leuprorelin, paclitaxel plus bevacizumab, eribulin, EC therapy, and docetaxel. However, both the primary lesion and liver metastasis increased. In particular, the increase in primary lesion size was remarkable, and the QOL significantly reduced due to bleeding and exudation. Biopsy performed during docetaxel treatment revealed metaplastic/squamous cell carcinoma(ER-, PR-, HER2 0, Ki-67 90-100%)histopathological findings. BRCA and microsatellite instability tests were negative, and PDL1 expression was less than 1%. Although Mohs ointment was used, tumor bleeding, exudate, and stink were poorly controlled, and the patient experienced painful symptoms due to the weight of the tumor. Therefore, left mastectomy plus pectoralis major muscle resection was performed. The patient died one month after the operation.