Since abnormal posture of the trunk due to aging decreases walking ability, the use of a trunk device is required to maintain and improve posture. Wearable assistive devices for the purpose of extending healthy life expectancy must be lightweight, have a simple mechanism, and have a drive system that cooperates with human movements. In our previous study, we developed an adjustable stiffness mechanism to correct the posture of the trunk. In this study, we propose a rotation mechanism that can support trunk rotation during walking and posture correction. The mechanism is a simple structure consisting of a bearing mechanism to reduce rotational resistance and a plastic block connected to pneumatic soft actuator bellows. Using a pneumatic soft actuator provides high adaptability to sudden human movements. This paper explains the outline of the proposed mechanism and the mathematical model of the actuator, followed by a description of the rotational stiffness adjustment method to support trunk rotation movements. Finally, the validity of the proposed mechanism and the effectiveness of the stiffness adjustment method are described based on the results of evaluation experiments.