PURPOSE: To keep airtight a transfer portion and to prevent diffusion of dust by disposing an outer cover for covering the transport surface and a flexible inner cover on a transfer portion of a boom conveyer capable of turning and taking a derricking motion, and disposing a seal member on the relative slide portion of the outer cover.

CONSTITUTION: When a transported article 2 is transferred from a conveyer 1 to a boom conveyer 3, the boom conveyer 3 is forced to take a derricking motion centering around a horizontal pin 8 by a hydraulic cylinder 10, whereby a rubber cover 17 is displaced without overstrain, with an opening portion 20 covered above a skirt 18. A front cover 12 and a rear cover 13 are respectively pivoted on hinges 15, so that the respective forward end rollers 14 are rolled on the skirt 18 to follow the movement of the boom conveyer 3. The contact portion between the central part cover 11 and the skirt rubber are slided airtightly. Accordingly, the boom conveyer 3 is forced to rise and fall with the interior of the transfer chute portion of the boom conveyer 3 kept airtight.