We report a rare case of multiple myeloma with biclonal gammopathy (IgG kappa and IgA lambda type) in a 58-year-old man with prostate cancer who presented with lower back pain. Through computed tomography (CT) imaging, an osteolytic lesion at the L3 vertebra and an enhancing lesion of the prostate gland with multiple lymphadenopathies were found. In the whole body positron emission tomography-computed tomography (PET-CT), an additional osteoblastic bone lesion was found in the left ischial bone. A prostate biopsy was performed, and adenocarcinoma was confirmed. Decompression surgery of the L3 vertebra was conducted, and the pathologic result indicated that the lesion was a plasma cell neoplasm. Immunofixation electrophoresis showed the presence of biclonal gammopathy (IgG kappa and IgA lambda). Bone marrow plasma cells (CD138 positive cells) comprised 7.2% of nucleated cells and showed kappa positivity. We started radiation therapy for the L3 vertebra lesion, with a total dose of 3,940 cGy, and androgen deprivation therapy as treatment for the prostate cancer.