A wheelchair is an effective transportation for physically challenged and elderly persons. A manual wheelchair is affordable and is also useful to prevent a muscular power from lowering, but is desired to correspond to driving on slopes. In particular, when a wheelchair runs on a cross slope, it turns to the lower side, and there is a risk causing to an accident. Then, it is necessary to add a large force to the lower side wheel in order to remove the load by the slope and to make the wheelchair go straight on. However, the operation is a difficult task for a user. The purpose of this study is to develop a control device that adequately assists according to the angle of the inclination and enables the user always to roll the wheelchair with the similar feeling on level ground. Through running experiments using the prototype wheelchair mounted with the control device, the effectiveness was confirmed.