A 60-year-old man visited our hospital because of the incidentally found mass of the rib on chest radiography. Chest X-ray showed expansile bony hypertrophy on left 5th rib and bone setting of the computed tomography scan of chest revealed 4.2×2.5 cm sized, elongated bony expansion with geographic radiolucent lesion in the medullary cavity and cortical thinning. Technetium-99m bone scintigraphy showed diffusely increased radioactivity along the left 5th rib. We present this case to discuss about a possible differential diagnosis in this type of lesion.