<p>PURPOSE: A construction method of an underground tunnel is provided to rapidly construct an underground tunnel by inserting a support structure into a square pipe assembly and placing concrete inside the square pipe assembly. CONSTITUTION: A construction method of an underground tunnel is as follows. A slab(100) is formed as follows. A reference square pipe is inserted into the ground. A square pipe assembly(40) for a slab is formed by forming connection square pipes on both sides of the reference square pipe. A support structure(41) is inserted into the inner space of the square pipe assembly. Concrete is placed in the inner space of the square pipe assembly and cured. Walls(200) are formed on both sides of the bottom of the slab. The inner space formed by the slab and the walls is excavated, and floor concrete is placed in the excavated tunnel part. The inside the tunnel is finished.</p>