A 35-year-old woman was referred to our hospital because of an abnormal finding on a chest X-ray film. Chest and abdominal computed tomography (CT) revealed a 25 × 5-mm tumor in the anterior right third rib as well as punched-out lesions in the vertebral bodies and iliac bones. Positron emission tomography (PET)/CT revealed accumulation in the right third rib and lumbar vertebral bodies. From these imaging findings and the lumbar magnetic resonance imaging (MRI) findings, we suspected a case of multiple myeloma. We performed histological bone marrow examination, serum protein electrophoresis, and urine immunoelectrophoresis. The patient was finally diagnosed with a Bence-Jones-type multiple myeloma. To our knowledge, this is a rare case of multiple myeloma in a young adult, discovered on the basis of an abnormal finding on a chest X-ray film.