arm 19 and has a free end extending toward the upper thread wiper 18 the free end of the elastic portion 313 extends out of the second inserting portion 310 and is formed with a positioning groove 314 for holding one end of a spiral spring 34 and another end of the spiral spring 34 is pushed against the key 32 at the opening end of the second inserting portion 310 of the positioning sleeve 31 is formed a u shaped second stop portion 315 which is located above the free end of the elastic portion 313 to restrict the key 32 the elastic portion 313 is provided at a bottom surface thereof with a triangle protruding engaging portion 316 to be elastically engaged with the first stop portion 194 of the wiper arm 19 as shown in fig9 the key 32 is a rectangular block which is formed with a first chamber 320 which opens toward the positioning sleeve 31 for accommodation of the spiral spring 34 and the free end of the elastic portion 313 the spiral spring 34 pushes against the bottom of the first chamber 320 to provide an elastic force for separating the key 32 and the positioning sleeve 31 from each other at an upper edge of an opening end of the first chamber 320 is formed an extending portion 321 which extends toward the second stop portion 315 and at a distal end of the extending portion 321 is formed a stop flange 322 perpendicular to the extending portion 321 at the corner defined.