To surely improve operability in mounting and demounting and improve maintenance property such as adjustment of a gap with certainty by easily inserting connection pins into a boss and each bracket.

A bush 21 is press-fitted into each of boss cylinders 20 arranged on both sides of an arm boss 18. A pair of brackets 11 is arranged so as to sandwich the arm boss 18 from both sides in an axial direction. First and second connection pins 23 are inserted into pin through-holes 11B of the brackets 11. A tapered portion 23A on the side of a leading end of the connection pin 23 is slidably inserted into a tapered hole 21A of the bush 21. The connection pin 23 is fixed to the bracket 11 through a flange 23C by means of a fixation bolt 24. The arm boss 18 is rotatably connected to the bracket 11.