PURPOSE: To make a high space unnecessary by supporting a bridging plate by a turning cylinder at an interval from the position of a hinge while sinking the position of the hinge when the bridging plate is pivoted at a high place at a vertical rest position.

CONSTITUTION: When a bridging plate 18 is pivoted at a high place and guided to a vertical rest position, an operating cylinder 21 is operated and a slide 16 is moved towards a lower section. The bridging plate 18 is pivoted centering around a hinge in the upper section of a turning cylinder 20. The turning cylinder 20 is pivoted in the direction towards a support 12 in the direction of the arrow 22'. The slide 16 reaches a position (a) while the place 19 of the hinge of the bridging plate 18 is sunk downwards, and the turning cylinder 20 runs parallel with the support 12.