Pneumatic artificial muscles have many advantages. They have high force mass ratio, high compliance and simple〓structures. Especially, the flexibility contributes to compose novel mechanisms. The flexibility of pneumatic artificial〓muscles releases us from the spatially strict design required for rigid mechanical elements: it could actualize compact〓mechanisms made of a fewer parts. OurresearchgroupdevelopedathinMcKibbenactuator. ThethinMcKibbenactuators〓is more flexible than conventional McKibben actuators. Authors succeeded in manufacturing novel soft mechanisms that〓are made of only braided artificial muscles. We aim to establish a design method for these novel mechanisms. We already〓have fabricated a cylindrical mechanism made of helical muscles. In this paper, we report a geometric model for the〓cylindrical mechanism. We focus on only deformation of a side surface in the coordinate system of muscles to simulate〓the movements of the cylindrical mechanism. The movement was verified by an experiment, and besides, it was simulated〓successfully in the geometric model that was created. The deformation of height and radius directions changed according〓to muscles alignment.