We developed and evaluated a functional electrical stimulator with a built-in feedback mechanism to suppress lateral knee joint sway during walking, which is a cause of pain in osteoarthritis. The proposed system detects the gait cycle with a pressure sensor placed on the sole of the foot. An angle sensor detects the lateral sway of the knee joint, and an electrical stimulus boosted by a step-up chopper circuit is applied according to the timing of walking. Then, the system feeds back the knee joint's lateral sway at each step and adjusts the amount of electric stimulation to suppress the knee joint's lateral sway and transform it into a desirable movement. To confirm the effectiveness of this device, we tested it on six patients with severe knee osteoarthritis and measured the changes in the knee joint adduction angle and adduction moment using a three-dimensional motion analysis device. We found that wearing the device improved the knee joint adduction angle and adduction moment and confirmed its effectiveness.