A case with post-traumatic cholesteatoma of the external auditory meatus extending into the mastoid is reported. A 27-year-old male visited our clinic with complaints of left otorrhea and hearing impairment lasting six months. He had suffered ahead injury three years previously. The left external auditory meatus was filled with otorrhea and debris, and an audiogram revealed slight conductive hearing loss. Computed tomography revealed a fracture line on the bony external auditory meatus and bone resorption at the medial part of the fracture near the eardrum. A soft tissue density mass was observed from the site of bone resorption to the mastoid antrum. During surgery, we found a temporal bone fracture filled with scar tissue, to which the external auditory meatus adhered. At the posterior part of the external auditory meatus medial to the fracture, the epithelium of the external auditory meatus had intruded into the antrum to form a cholesteatoma, which had extended into the whole mastoid cavity. Constriction of scar tissue at the fracture site during healing and a loss of ventilation at the mastoid were suggested to be involved in the pathogenesis of this cholesteatoma.