To regulate a hitch load applied to a coupler to an appropriate value, by energizing a hitch coupler so that the rotational position of the hitch coupler is positioned in a neutral position when the hitch load applied to the coupler of a tractive device becomes the appropriate value.

An indicator bracket 25 is fixed to a hitch member 8, and an indicator unit 26 is fixed to this indicator bracket 25 by a bolt 27. The front surface of an indicator housing 28 is opened, and a display window 29 is opened in the left side surface of the indicator housing 28. Two washers 35, 36 are mounted on an indicator stem 31 so that an instruction plate 32a is vertically sandwiched therebetween. Springs 37, 38 are respectively elastically attached among the upper and lower side washers 35, 36 and the upper and lower surfaces of the indicator housing 28. A hitch coupler 11 is energized by these two upper and lower springs 37, 38. That is, the instruction plate 32a is stopped in a neutral position under a condition that the springs 36, 37 are mutually pushed and balanced.