The ankle over-pronation causes the mechanical overloading transferred to proximal areas (i.e. knees or hips) over time. Thus, the over-pronation is recognized as a contributory factor in a wide variety of musculo-skeletal pathologies in lower extremities. Commonly, over-pronated ankles are treated using specially designed insoles that support medial heels and correct the posture of lower limbs. However, the biomechanical effects of the insoles are not yet fully understood, so there still are controversies whether such insoles really have clinical significance. In this study, in order to verify the effects of insoles and determine the best shape of the insoles, we examined how the medio-lateral knee joint reaction force changes due to insole conditions through a case study about a subject. As a result, it is revealed that the medial heel post, which drastically reduced the peak medio-lateral knee joint reaction force, has significant effects on the gait of the over-pronated patients. However, in case that the arch support is combined together, the positive effect of the medial heel post may rather decrease.