this may enable a contact surface of the roller contacting the drum to have a wider area therefore a stress applied to the outer circumferential surface of the roller decreases to prolong the lifespan of the roller preferably the roller includes a roller frame 54 configured to insert the roller shaft 51 therein by using the ball bearing 53 and an outer wheel portion 55 mounted to the roller frame 54 and configured to encompass an outer circumferential surface of the roller frame 54 in this case an outer circumferential surface 55 a of the outer wheel portion 55 directly contacting the drum is formed to be flat thus to have a wider contact area as the contact portion of the outer circumferential surface 55 a contacting the drum is formed to be flat the stress applied to the roller is distributed to prolong the lifespan of the roller a plurality of concave convex portions are formed on an outer circumferential surface of the roller frame 54 and a plurality of concave convex portions engaged with the concave convex portions are formed on an inner circumferential surface of the outer wheel portion 55 as the concave convex portions of the roller frame 54 are coupled to the concave convex portions formed on the inner circumferential.