Atlanto-axial instability caused by the pyogenic infection complicated after tonsillectomy under local infiltrative anesthesia was observed in a 29-year-old male patient. Severe neck pain and stiffness with high fever were developed from the day after operation. Anterior subluxation of atlantoaxial joint could be observed 2 weeks later. The interval between anterior tubercle of atlas and odontoid process and the space available for spinal cord were 11 mm, each. After trial of conservative treatment with antibiotics and cervical collar for two months, posterior atlanto-axial arthrodesis was performed for the persisting neck pain and subluxation. Coagulase negative staphylococcus was isolated from the granulation tissue in the epidural space which was taken during operation.