PURPOSE: To make it possible to easily mount a gate opening/closing auxiliary device on the body member of any type of truck by making a tool composed of a graph for indicating the distance between the center of a gate hinge and the core of a mounting bolt, and a seal gauge which, supports the position of the mounting bolt when it is stuck to the body member so as to match the marking-off line on the body.

CONSTITUTION: A line 14 indicating a bolt hole, a hinge center line 15 and right and left cross sill outer lines 16 are drawn on one side of a seal gauge 13 made of a transparent plastic sheet and the other side of the seal gauge 13 is given a coat adhesive. The center line of a hinge 6 and the marking-off line of the cross sill outer lines are then marked on the outer surface of a vertical member 4 of a frame, and the hinge center line, which equals W the value obtained from the graph indicating the distance between the center line of the hinge 6 and a mounting bolt, equalling the step amount of the gate, is matched with a marking-off line 9. Simultaneously, the seal gauge is adhered on the body member after matching the cross sill outer line 16 with the marking-off line 9 on the cross sill outer line, a hole is drilled in the vertical member 4 matching the line 14 indicating the bolt hole and a bracket 2 is fitted by a bolt 3.