Various welfare devices have been developed by the influence of population aging society. They are required to support caregivers as well as cared people these days, because the burden of nursing care is increasing. A method to realize caregiver's power-assist control for an electric wheelchair is proposed in this paper. This method enables power-assisted wheelchairs to adapt not only to flat roads, but also to roads with steps. Instead of a force sensor, a reaction torque estimation observer is constructed to detect the human torque input into the wheelchair. An inclination sensor is used to detect the step, and to determine the control method while passing over the step. Simulation and experiment were carried out to show the validity of the proposed method.