We present a very rare case of metachronous triple cancers, including small cell carcinoma of the lung, as well as prostate and male breast cancer. To our knowledge, this is the first documented case of its kind. A 64-year-old man was referred to our hospital with left nipple retraction. He had previously undergone lobectomy of the right lung as treatment for small cell lung cancer at 57 years of age, and at 61 years of age, he had undergone prostatectomy and bilateral orchiectomy for prostate cancer, histologically determined to be moderately or poorly differentiated adenocarcinoma. Physical examination identified a painless irregular hard tumor in the left breast. Ultrasonography and magnetic resonance imaging (MRI) showed a nodular mass, and fine needle aspiration cytology of the mass revealed adenocarcinoma. Modified radical mastectomy was performed. Histological examination revealed that the breast tumor was scirrhous carcinoma, t1, n0, m0, stage T. Immunohistochemistry demonstrated that the prostate tumor was positive for prostatic specific antigen (PSA) and negative for estrogen receptor (ER), while the breast tumor was positive for ER and negative for PSA. Primary breast cancer was diagnosed. At present, 1 year and 8 months after surgical removal of the breast cancer, the patient has had no recurrence of breast cancer, small cell lung cancer, or prostate cancer. We discuss the possible causes of the triple cancers in this case with reference to the literature.