We treated a patient with a pseudoaneurysm caused by core needle biopsy (CNB), in which both the cancer and the aneurysm were excised by breast conservation therapy. A 51-year-old woman attended a local hospital because of a 25-mm mass in the upper outer quadrant of the right breast. CNB was performed, and brisk bleeding occurred at the biopsy site. Immediate hemostasis was achieved with direct manual compression. CNB detected fatty tissue, and a diagnosis could not be made. When she presented at our hospital 6 weeks later, there was a 25-mm pulsating mass at the biopsy site. Color-flow Doppler US and dynamic MRI showed a breast tumor and pseudoaneurysm formation. For the purpose of diagnosis and treatment of the breast tumor and pseudoaneurysm, lumpectomy of the right breast was performed. Histological diagnosis was papillotubular carcinoma and pseudoaneurysm. Although this condition is relatively rare, it is important to be aware of the possibility of complications, such as pseudoaneurysms, which require treatment.