For reducing the risk of pressure sores caused by a long period of sitting in a wheelchair, we have developed a depressurization motion assistance system that is low in cost and suitable for practical use. Our developing system consists of a seat cushion that the patient sits on and four air cells that can lift or incline the seat cushion. In this paper, our key ideas are two. One is mechanical design for practical use. We realize a thin mechanism that enables easy implementation on a standard wheelchair. For realizing this thin design, we developed a tilt mechanism using the elasticity of acrylic resin and a controller that uses only pressure sensors for estimating lift height and inclination. The other topic is an assistance control scheme based on the patient’s depressurization operation for increasing rehabilitation performance. For realizing the proposed control scheme, we analyzed hip depressurization operation by nursing specialists and used results for estimating the patient’s condition.