PURPOSE: To upgrade efficiency by mounting a timbering conveyed by means of the rotation of a rotation bracket and the extension of a gripper jack after moving a gripper block forward while a gripper, which is provided on a tunnel excavator, supports the timbering.

CONSTITUTION: A tunnel excavator is supported by a leg 14 extended downward while the tunnel excavation operation is being suspended. After detaching a gripper 13 from the tunnel wall by contracting a gripper jack 8, a timbering 20 is fitted to a grip device 22 by rotating a rotation bracket 15 so as to bring the grip device 22 of the gripper 13 in the position where the timbering 20 can be gripped. The timbering 20 is then moved forward by contracting the gripper jack 8 and a propulsion jack 9 and, after the rotation bracket 15 is rotated to a specified position, the timbering 20 is linked and assembled to timberings already assembled by extending the gripper jack 8. Timberings can thus be assembled without difficulty.