Petrositis is rare occurrence. We present a 5-year-old boy with petrositis due to mucoid type of Streptococcus pneumoniae, which caused inflammation into perapharyngeal space. He complained of left otalgia. CT showed petrositis with inflammatory change in parapharyngeal space. Mastoidectomy was performed, and abscess was drained. Subsequently we administrated antibiotics of carbapenem and clindamycin. Mucoid type of Streptococcus pneumoniae was isolated in pharynx. The patient recovered uneventfully. We considered that the drainage and appropriate administration of antibacterial agent would be very useful for acute otitis media with petrositis, especially due to mucoid type of Streptococcus pneumoniae.