We encountered a 36-year-old female who developed extracranial vertebral artery dissociation (EVAD), which is a rare condition in Japan. The disease developed with sudden severe right side neck pain and right upper arm pain. No abnormality was observed in the spinal canal, intervertebral discs, spinal cord, or nerve roots on MRI of the cervical spine, but dilation and double lumen were noted in the vertebral artery (VA) at the C4/5 level. The presence of aneurysm was confirmed by VA angiography, and EVAD was diagnosed. The symptoms were resolved by conservative treatment of pain, and the aneurysm shrank and disappeared with time on VA angiography. Even though neck and upper limb pain are the only clinical symptom and there is no brain dysfunction, such as disturbance of consciousness and vertigo, when symptoms develop acutely, this pathology should be included in differential diagnosis and orthopedists should carefully observe the vertebral artery, as well as the spinal canal, in evaluating cervical MRI.