A 71-year-old man with dysuria was referred to our hospital. The level of serum prostate specific antigen was slightly elevated (4.66 ng/ml), and digital rectal examination revealed a stony hard prostate mass. We performed a transrectal prostate biopsy because malignancy was suspected. Histological examination revealed leukemia-like cells, and bone-marrow examination (aspiration) was performed to determine the location of the original lesion. However, no leukemia-like cells or any other form of malignant cells were identified. Clinical imaging confirmed the absence of any other lesions, and granulocytic sarcoma of the prostate was subsequently diagnosed. We treated the cancer with radiotherapy at a dose of 40 Gy and dysuria improved. Furthermore, significant reduction in prostate volume was confirmed. Four months after initial presentation, the patient developed acute myeloid leukemia [M2 by French-American-British classification]. Induction chemotherapy was initiated, and the patient was successfully induced to complete remission. Twenty months later, the patient showed relapse. Despite salvage chemotherapy, he died of brain hemorrhage twenty-four months after complete remission.