To surely return a guard arm into a case, in a guard arm device of a type shaking out the guard arm of which the base end is supported rotationally movably and tiltably.

The width of the opening 8 of a front plate 7 through which a guard arm goes in and out is set larger than the shake of the extreme end of the guard arm at least in ordinary return motion. A partition plate 11 is bent into a trapezoidal section in front view, the length of the top side of the trapezoid of the part at least receiving the extreme end of the guard arm is set into the degree of the width of the guard arm, and the length of the bottom side is set to be the dergee of the width of the opening of the front plate.