The front end and rear end of a belt-shaped member having a predetermined length are grasped by front end grasping pawls and rear end grasping pawls, respectively, movable in synchronism with and independently from each other when required. The front and rear end grasping pawls are moved in synchronism with each other until the front end of the belt-shaped member arrives at the forming drum. After the front end of the belt-shaped member has been pressure-joined with the forming drum, only the rear end grasping pawls grasping the rear end of the belt-shaped member are moved to attach the belt-shaped member to the circumference of the forming drum. The length of the belt-shaped member is modified to a predetermined length or the circumferential length of the forming drum while the front and rear ends are grasped by the front and rear end grasping pawls, respectively, or the belt-shaped member is being attached to the forming drum.