A rare case is presented of a synovial cyst located at the level of C3-C4 that caused cervical myelopathy and that was preoperatively diagnosed by facet arthrography. A woman in her late seventies experienced muscle weakness and numbness in her right upper extremity and gait disturbance. MRI revealed an extradural lesion located dorsolaterally on the right side of the spinal cord at the level of C3-C4. CT facet arthrography revealed continuity of the extradural lesion with the right C3-C4 facet joint and infiltration of contrast medium into the lesion. Postoperatively, histological examination of the cyst showed fibrous tissue with calcium deposits and the presence of synovial lining. Preoperatively, cervical synovial cysts are often difficult to distinguish from other extradural lesions. In this case, facet arthrography allowed the preoperative determination of communication between the extradural lesion and the facet joint, leading to the diagnosis of a synovial cyst.