Unilateral dislocation or subluxation of a facet in lumbar spine is extremely rare, so it has been often neglected. The mechanism of injury is hyperflexion and distraction forces. As for the treatment of lumbar facet dislocation and subluxation, open reduction and internal fixation by the posterior approach has been recommended because the injuries are resistant to closed reduction and they may cause chronic instability. We report a case of neglected unilateral subluxation of facet joint in lumbar spine of multiple trauma patient, which was treated by open reduction and internal fixation with posterolateral fusion. In the multiple trauma patients, it is mandatory to the meticulous diagnosis and treatment.