An excavating drive (22) movable backward/forward is accommodated in a turnable body (9) which in turn is turnable about an axis perpendicular to an axis of and in a skin plate (7). A cutter device (38) ahead of a rotor (22) in the excavating drive (22) is composed of a center cutter (40) supported by the excavating drive (22) and a plurality of face plate shaped expansion cutters (44) fitted to an outer periphery of the center cutter (40) for pivotal movement backward/forward. Tunneling is effected by the expanded cutter device with the expansion cutters (44) being pivoted forward. Upon replacement of bits (35; 35), the expansion cutters (44) are pivoted backward for contraction of an assembly of them and the cutter device (38) is accommodated in a turning trajectory (R) of the turnable body (9) so that the turnable body (9) is turned backward for replacement of the damaged bits (35; 35) backward.