An attaching wall 82 is interposed between first and second walls 81a, 81b. A banding band 9 is inserted from a first gap 83a provided between the attaching wall 82 and the first wall 81a to a second gap 83b provided between the attaching wall 82 and the second wall 81b, there by the banding band 9 is attached to the attaching wall 82. A through-hole 8 is provided on the attaching wall 82. A first guide wall 85a inclined toward the attaching wall 82 as extended toward the second wall 81b is arranged at a position facing the through-hole 84. Further, a second guide wall 85b inclined toward the attaching wall 82 as extended toward the second wall 81b is arranged at a position facing the second gap 83b.