In 1940 the exercise program called "Single -Boot Technique" was applied on injuries of the knee joint. This technique was the first scientific approach to the rehabilitation of the knee joint in the pre and post operation stage. Until the arrival of isokinetic exercise machines such as the Cybex Machine in 1975, several case studies of rehabilitation were reported by Carl. F, Klaft. William and others. In these studies preoperation exercise programs were considerd very important and the details of completing the postoperation exercise were presented. The use of the arthroscopy for the operation of the knee has shorten the periode of rehabilitation. This study deals with the method of rehabilitation of the knee after having an arthroscopic operation of the medial meniscus or the A.C.L. After the introduction of the Cybex Machine in 1975, rehabilitation programs and its evaluation were based totally on the information of the Cybex Machine. But the experience of the past decades made it possible to evaluate the condition of the knee in the early postoperation stage without the Cybex Machine. Moreover, the use of the Cybex Machine in the first periode of rehabilitation after having an operation of the A.C.L. could cause secondery injuries on the quadriceps and other related muscles because of the strength of the exercise. It is important to begin the rehabilitation with physical therapy and isometric exercise so that the extention of the knee joint is completed in an early stage. The rehabilitation program should shift from isometric exercises to isotonic exercises and isokinetic exercises using the Cybex Machine should be applied in the final stage. This is to makes it possible to take the merits of both of isometric and isotonic exercises. While the exercise using the Cybex Machine is too strong in the restoring periode, it should be recommended to use the Cybex Machine from around the 12th week (3d month).