A 51-year-old man had progressive hearing loss over more than 15 years. He had bilateral sensorineural hearing loss (SNHL). Computed tomography (CT) showed extensive bilateral demineralization of the cochlear capsule, which is characteristic of diffuse cochlear otosclerosis. Three-dimensional fluid-attenuated inversion recovery (3D-FLAIR) of magnetic resonance imaging before enhancement revealed high signals in the cochlea and vestibule. Postcontrast 3D-FLAIR revealed enhancement of the basal turn of the left cochlea. This is the first published case of the breakdown of the blood-labyrinth barrier in a patient with cochlear otosclerosis. Our findings suggest that the breakdown of the blood-labyrinth barrier is associated with a part of SNHL in cochlear otosclerosis.