PURPOSE: To expedite an assembling work by providing a multijoint erector arm symmetrically on the rotating drum of the erector device of a shield excavator, and assembling the segment of an adjacent tunnel by the central erector.

CONSTITUTION: At least a pair of multijoint arms 7, 8 having segment holding parts 9 on the top ends are provided bilaterally on the rotating drum 5 of an erector device 2 rotatable in the central tunnel of a tunnel having a three-throw deformed composite sectional form. Segments A1, B1 of the central tunnel are assembled by the erector 2, and segments A2, A3 of the adjacent tunnels carried to the central tunnel are held by the holding part of the arm, and mounted. After the mounting of the segments of the adjacent tunnels is finished, strut segments CL, CR are mounted on both sides. Thus, the erector device 2 can be provided with a wide usability, and made compact, and the assembling work of segments can be efficiently performed.