An upper arm portion and a lower arm portion are connected to each other by a substantially cylindrical elbow joint, and are bendable between an extended position in which they are extended into a substantially straight line and a bent position in which they are bent forwards from the extended position. The position of an axis of the elbow joint is offset forwards from the center of the width of the upper and lower arm portions, and slants inclined toward the elbow joint are formed on rear surfaces of the upper and lower arm portions connected to the elbow joint. Thus, when the lower arm portion is turned toward the bent position, a foreign matter can be prevented from being sandwiched between the upper and lower arm portion at a location in front of the elbow joint by displacing the position of the axis of the elbow joint forwards. In addition, when the lower arm portion is turned rearwards toward the extended position, a foreign matter can be prevent from being sandwiched between the upper and lower arm portion at a location in the rear of the elbow joint by forming the slants.