In this paper, the authors develop a 2-DOF power assist suit (PAS), namely Tornado. Our developed PAS assists power for twisting motions in addition to lifting and lowering motions in environments of agriculture, fishery, factory, and so on. Tornado PAS not only assists power to some motions but also protects our lower back by limiting high-risk motions which causes lower back injuries. The authors apply a differential gear box mechanism to achieve 2-DOF power assist. The prototype of 2-DOF PAS is implemented by comprising two DC motor, two rotary encoders, two current control motor drivers, 2-channel encoder counter board, 2-channel DIO board, battery, and Raspi4 controller installing Linux OS. Although Tornado PAS assists power according to an external force, our system do not use any force sensor. We apply a disturbance observer (DOB) to estimate such an external force. Thus, Tornado is characterized as follows: (1) a unique mechanism using a differential gear box, (2) not only power assist but also prohibiting dangerous motions by computer control, and (3) force sensorless power, i.e., force, assist. In this paper, the authors model Tornado, and establish equation of motion in order to simulate various types of control methods and observers. We experiment power assist of twisting motions based on some position tracking control. Results this control and values estimated using DOB are discussed and evaluated for effective power assist for twisting motion.