Congenital atlanto-occipital assimilation is frequently accompanied by basilar invagination and C1-2 instability. Occipitocervical fusion is required if these conditions cause neurological symptoms. Recently posterior fusion using occipitocervical screw fixation in the extension position has been introduced for the simultaneous decompression of the cervicomedullary junction compressed by basilar invagination, a reduction of atlantoaxial subluxation, and rigid fixation. However, it is a technically demanding procedure, and there is no domestic report of an experience with this procedure. We report a case of a patient with C2-3 block vertebra and C3-4 retrolisthesis in addition to congenital atlanto-occipital assimilation, basilar invagination and C1-2 instability. The patient underwent posterior fusion using occipitocervical screw fixation in the extension position.