PURPOSE: To improve safety by making adjustable the height of a carrier roller supporting a conveyor belt between a main body and a boom which rises and falls.

CONSTITUTION: One end of a medium frame 21, provided between a main body 15 and a boom 20, is installed rotatably up and down on the main body 15 by means of a pin 22, while the other end is rotatably installed on the boom 20 by means of a pin 23 through a long hole of the medium frame 21. In such a condition as the boom 20 rises, a conveyor belt 24 is linearly and evenly supported by means of a roller supported by carrier frames 25, 27, 29 provided upright on the main body 15. When the boom 20 falls downward, following its rotational movement, the medium frame 21 also makes a downward rotational movement with a pin 22 on the main body 15 as a fulcrum of rotation. Thus, since the conveyor belt can be certainly supported by means of the carrier roller without floating, its safety is improved.