PURPOSE: To reduce the term of works and the cost of construction, and to prevent the displacement of a ring by filling the air gap of the outer circumference of a precast concrete block unified with a reinforcing bar with cast-in-place concrete and supporting the reinforcing ring to the inside of the air gap.

CONSTITUTION: An air gap 3 is assembled to the tail section of a shield body 1 to a ring shape at a required clearance. Reinforcing bars 4 are unified with precast concrete blocks 2, and dowel reinforcements 5 are projected at regular intervals. A plurality of the reinforcing rings 6 of semicircular arc members, one ends of which are connected rotatably by hinges 8 and to the other ends of which expansion jacks 9 are installed and in which two rings are paired, are mounted on the inner circumferential surface sides of the concrete blocks 2. Concrete 13 is placed and filled into the air gap 3 from a placing hole 12, and pressed by a press ring 11 by operating a press jack 10. Accordingly, the concrete blocks 2 are assembled to a ring shape in place of forms, thus improving the finish of the internal surface of a tunnel.