A 38-year-old man presented with untreated congenital kyphosis associated with progressive spastic gait. To prevent progression of the spastic paralysis, rigid correction of the severe spinal deformity arising from the congenital kyphosis was performed by one-stage posterior closing-wedge osteotomy, without occurrence of neurological complications. Progression of the paralysis has not been identified for 30 months after the operation and a slight improvement in gait was recognized. The current case is categorized as type I deformity of congenital kyphosis in the upper thoracic spine, which is normally treated surgically before the adolescent growth phase begins.