To construct a branch tunnel economically and in a short period by driving a master-slave shield machine into a main track tunnel, and launching only a slave machine from the inside of the main track tunnel for constructing a branch tunnel.

A thin steel segment 31 is used in the penetration zone of a branch tunnel 2 at the intermediate position of a main track tunnel 1. The though-hole parts of the steel segment 31 are combined so as to be overhauled to suit the facing shape of a branch shield machine 21. This branch shield machine 21 is made to near the zone of a branch part structure 3. Then, the steel segment 31 at one through-hole part is overhauled and an entrance packing is fitted instead. Then, a solidifying material 32 around the branch part structure 3 is cut with the branch shield machine 21 for causing the branch tunnel 2 to penetrate. The through-hole part 33 at the other side is similarly processed. Thereafter, the surrounding natural ground of the branch part of the main track tunnel 1 and the branch tunnel 2 is filled with the solidifying material 32 from the inside of the steel segment 31.