A method and an apparatus for connecting halves of a shield tunnel under the ground, which are preferably applied to an operation for digging the portion of the natural ground in which a tunnel is to be made, from the opposite ends thereof by using a couple of shield excavators, and then connecting the two halves of the tunnel at an intermediate part thereof, are disclosed. In an operation for connecting halves of a tunnel under the ground, it is important to reliably seal the tunnel against the earth and water pressure, which is imparted to the tunnel from that part of the natural ground which is in the vicinity of the joint part thereof, and prevent the water from entering the tunnel. In order to meet these requirements, this connecting apparatus provides an intrusive ring in one shield excavator, and a chamber, in which the intrusive ring can be accommodated, in the other. The two shield excavators are disposed in opposition to each other with a narrow space left therebetween, and the intrusive ring is then moved forward and inserted into the chamber in the opposed shield excavator. As a result, the opposed parts of the two excavators are sealed with this intrusive ring. These shield excavators are then disassembled, and the wall surfaces of the joint parts including that of the intrusive ring are lined to complete an underground connecting step. According to this method using an intrusive ring, the sealing of a joint part of a tunnel agaisnt the earth and water pressure at that part of the natural ground which is in the vicinity thereof and the prevention of the entry of water into the interior of the tunnel can be done reliably.