An 11-year-old girl who had numerous exostosis was reported. Range of motion of her right knee joint was restricted in flexion by the multiple exostosis of the distal portion of the femur and the proximal portions of the tibia and fibula. Surgical resection of the tumor around the knee was performed and histological findings of the tumor showed benign osteochondroma. Her father and sister were also involved in the same disease and the family history was investigated in the hereditary aspects.