PURPOSE:To lessen an occupation area on the ground to reduce the effects to surface traffic during the construction period and, at the same time, to reduce excavated earth and to dissolve environmental problems such as an earth dumping place, etc. CONSTITUTION:A working base 14 is provided in parallel with an underground tunnel 10. After that, column bodies such as a pipe roof 15, etc., constructed the underground tunnel 10 from the working base 14 are arranged in the extending direction of the tunnel to provide an preceding underground roof 16. The ground covered with the preceding underground roof 16 is excavated and withdrawn through the working base 14 and, at the same time, a widened section 12 of the tunnel such as branch/junction, etc., of the underground tunnel is constructed in a space made thereby.