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Abstract— Today, a welfare pneumatic equipment to support a nursing care and a self-reliance of the elderly and the disabled is actively researched and developed by many researchers. In the previous study, a portable rehabilitation device with a larger moving area which can give passive exercise for human shoulder was proposed and tested. As an actuator of the device, an extension type flexible actuator was tested. The actuator can extend more than twice of its original length. However, a flexible displacement sensor that can cover the actuator’s moving area and be deformed according to the shape of the actuator has not been realized yet. In this study, various flexible displacement sensors using ultrasonic sensors for the actuator are proposed and tested. One is a displacement sensor that ultrasonic sensors are installed into the chamber of the actuator. As a result, it is confirmed that the sensor can not measure the displacement exactly under the condition when a higher supplied pressure is applied to the chamber. Therefore, a displacement sensor using a rubber tube and the ultrasonic sensor is proposed and tested as a sensor without influence of applied pressure. As a result, the sensor can successfully measure the displacement. However, the sensor requires the force to pull the tube. The pulling force causes local deformation of the actuator. Finally, the slide type displacement sensor without pulling force is proposed and tested. As a result, it is confirmed that the tested sensor can measure successfully and exactly.