The objective of this research is to inverted stabilization of an electric wheelchair with a tristar mechanism developed in our research group, which is necessary in the process of stair climbing and descending. Tristar wheel mechanism consists of a Y-shaped carrier and three wheels which are synchronized. Both the Y-shaped carrier and the wheels are controlled by two motors via a planetary gear mechanism. In the stair climbing process accomplished by rotating the Y-shaped carrier, it must pass through the two-wheel inverted state in which the wheelchair must stand by one wheel on each side. Therefore, in this study, a mathematical model of the wheelchair system is derived and an output zeroing control is designed based on the model to realize a two-wheeled inverted state. The effectiveness of the proposed method is demonstrated through numerical simulations.