A 58-year-old man presented with a 6-month history of progressive cervical myelopathy. Magnetic resonance imaging and computed tomography of the cervical spine revealed a bone tumor arising from the posterior arch of the atlas and osteophytes at a pseudoarthrosis between the tumor and the lamina of the axis, resulting in marked spinal cord compression. The patient's symptoms resolved after en bloc resection of the tumor and removal of the osteophytes. The histological diagnosis was osteochondroma. The primary cause of myelopathy in the present case was osteochondroma arising from the posterior arch of the atlas, but the osteophyte formations appearing at the pseudoarthrosis between the atlas osteochondroma and the lamina of the axis might also have contributed to the symptoms, which appeared when the patient was in his late 50s.