PURPOSE: To construct a branched tunnel even when the cross section of its main tunnel is small and construct both main tunnel and branched tunnel in parallel smoothly.

CONSTITUTION: A main tunnel 10 having two tunnel spaces 11 is constructed where each of the tunnels is formed with a post segment 122 respectively. There is installed an opening section communicated between the tunnel spaces 11 where no post segment 122 described above is installed at a branched location of the main tunnel 10 and a notched part is formed on a side opposed to the post segment on both sides. A shield machine 40 for branch tunnel construction is laid out directed at the penetration direction in the opening. The shield machine 40 is launched from the main tunnel 10, thereby constructing a tunnel 30 branched from the main tunnel 10.