With the aging of population in Japan, research of electric wheelchair for higher performance is getting more and more important. Power assist wheelchair is an example of electric wheelchair. Power assist wheelchair amplifies passengers maneuvering force, thus it is useful for elderly people who have weaker muscle force. And it is also helpful for aspect of health-maintenance because it requires using surviving ability of passenger. One big difficulty for wheelchair users is maneuvering in slope environment where gravity effect exists. In this research, "equivalent jerk disturbance" which is differential of estimated disturbance by observer is used for power assist control. Equivalent jerk disturbance is unaffected by gravity force, thus it is useful for wheelchair users to maneuver under slope environment.