Abstract— The progress of Japanese aging society is getting worse. Therefore, the development of rehabilitation devices and power assistive devices is desired from the viewpoint of quality of life. In such devices, it is required to use human-friendly soft actuators. In ideal, the soft actuator with both a longer stroke and a larger generated force is required. Therefore, we have developed a various type of flexible linear stepping actuators that can push and pull the flexible tube while changing the gripping position of the tube. In this paper, the improvement of the pneumatic chucks for gripping the tube to increase the generated force is described. The construction and the operating principle of the improved pneumatic chucks are described. Generated force of the tested chuck is investigated. As a result, the actuator with improved chuck can generate larger force that is almost same as the theoretical generated force without slip.