PURPOSE: To enhance the working efficiency by supporting a leading conveyor and a follower conveyor in such a way as movable and within frames slidable at two stages over and below, forming the forward path part of each conveyor as having a U-section with the over-part left open at all times, and the return path part as having a circular section.

CONSTITUTION: Frames 20A, 20B equipped with a moving means 21 and replaceable leg 22 are installed in positioning overlapped over and below. Each frame of conveyor 20A, 20B is divided into a forward path part 36 and a return path part 37, and the belt section is formed in U-shape in the forward path part 36 and cylindrical in the return path part 37. Debris in mud form or in lumps produced at a facing 2 in a tunnel 1 is carried by a working vehicle and cast onto a screen 4 on the loading side of the leading conveyor 3 and the muck having passed the screen mesh is transported by the conveyor while the lumps 5 failed in passing through fall on the side-wall side. A dump truck is reciprocated to/from a floor surface concrete placing part 7 in the tunnel, and the lumps 5 are carried out.