A user of a power wheelchair usually utilizes a joystick as an input device. However, when one cannot manipulate a joystick due to some kind of physical disability, an input device of the simple switch type replaces the joystick. The switching device is, in general, more inconvenient to use than the joystick, (e.g., when completing turnabout actions). Specifically, it causes one crucial problem when crossing a slope, where the user has to operate the switch more frequently so that the wheelchair continues to go straight. Therefore, a series of operations of the switch may put a strain on the user. This research aims to reduce the user's strain of operations on a slope, and proposes a support system which assists a user for driving a power wheelchair to go straight ahead on a slope by using a simple switching operation. Through the experiments with a prototype system, the effectiveness and the applicability of the proposed system have been confirmed.