It is often difficult for handicapped people using the electric wheelchairs to maintain their posture against external forces. Therefore, it is important to reduce the instability in driving the wheelchair under the various road conditions. In this study, the active level holding seat mechanism available for the general electric wheelchairs is proposed. In order to reduce the increase of seat height, optimal link parameters were designed to minimize the size of linkage. A prototype model of level holding mechanism was made practically. The effect of the proposed mechanism was evaluated by the running tests with electric wheelchair. As the results, the stability of the driver and wheelchair system increase by the decrease of the upper trunk displacement of driver using this seat mechanism.