PURPOSE: To form an enlarged width part in a tunnel by carrying out enlarged excavation with the length and with the angle of a boom changed, and thereafter by filling the concrete in the enlarged with part, and by grinding a part of the concrete after the concrete is cured to a prescribed degree of hardness.

CONSTITUTION: Enlarged excavation is carried out by changing the length and angle of a boom 10, having a cutter 11 for a natural ground excavator 1 in a zone to be enlarged of a tunnel. Then, a shield 3 is advanced by jacks 4 which obtain reaction force from segments 6. Concrete is filled in an enlarged width space defined in rear of the shield 3. After the concrete is hardened to a prescribed degree of hardness, a part of the concrete is cut so as to form an enlarged cutout.