PURPOSE: To diminish the operating physical force of a gate handle, and simultaneously lighten the gate handle, further suppress rub between a hook and a gate handle link, by proving a groove intersected to the gate handle link, on the pressure contact surface of the hook with which the gate handle link is brought in pressure contact.

CONSTITUTION: In a hook 9 arranged on the side surface of a back panel 5, a fixing flange part 12 is formed on one end part, a mounting hole 14 through which a mounting bolt 13 is inserted is opened, and a hook part 15 is fixed on the side part of the back panel 5 by the mounting bolt 13 in such a direction that the hook part 15 is projected from an opening part 17 opened in the arrangement part of the gate handle 6 of a side gate 3. While, the hook part 15 on the other end part is formed by being buckled in a direction transverse to the flange part 12, and a pressure contact surface 10 with which the abutting surface 11 of a silencer 8 is brought in pressure contact is formed on the buckled inside surface of the hook part 15. A groove 16 is formed in a direction transverse to the silencer 8 on the pressure contact surface 10. By this structure, coefficient of friction of the pressure contact surface 10 of the hook 9 can be increased.