A 64-year-old man was diagnosed with metastatic prostate cancer (cT3bN0M1b) and treated with combined androgen blockade. After two years and three months, he developed castration-resistant prostate cancer. Multiple lung metastases appeared after the administration of five courses of docetaxel and four courses of cabazitaxel therapy. Pulmonary metastases disappeared following rechallenge with docetaxel. Enzalutamide administration was initiated because docetaxel had to be discontinued due to adverse events. Although enzalutamide lowered the prostate specific antigen value, the patient staggered while walking and developed homonymous hemianopsia. Magnetic resonance imaging revealed a brain tumor. Although the brain tumor was considered to have metastasized from the prostate cancer, it was diagnosed as a primary central nervous system lymphoma using open-ended tumor biopsy. The brain tumor was eliminated with whole-brain irradiation. Thereafter, he has been treated with enzalutamide for 3 years without clinical progression of either disease.