We report a case of infection of a mastoid cavity after mastoidectomy had been performed for chronic otitis media with cholesteatoma. The infection was caused by Staphylococcus intermedius after a pet dog had licked the patient's ears. Bacterial strains from the dog's saliva and the otorrhea in the patient were confirmed to be identical by pulsed-field gel electrophoresis analysis. The possibility of an oral transmission route of S. intermedius from pets to humans should be noted.