A 67-year-old male came to our department with complaints of urinary retention and gross hematuria. The prostate specific antigen (PSA) level in the serum was elevated to 69.5 ng/ml. Thus a transperineal prostate biopsy was performed. The patient was diagnosed with prostate cancer, and lung and bone metastases were also revealed. Treatment for metastatic prostate cancer was performed for approximately 5 years with combined androgen blockade therapy followed by enzalutamide, docetaxel, estramustine, Ra-223 dichloride, estradiol, and then enzalutamide reintroduction. Thereafter, the patient presented with bilateral breast nodules and we referred him to our breast surgery department. Breast needle biopsy findings revealed breast metastasis from prostate cancer, that was not primary breast cancer. The patient underwent a bilateral mastectomy.