PURPOSE: To raise the assembling accuracy of an underground beam as well as reduce the amount of labor used by a method in which an L-shaped mesh part consisting of integrally combined bottom and side parts, a mesh part consisting of the other side parts, and a mesh part of the upper part are manufactured in a factory and then assembled in construction site.

CONSTITUTION: An underground beam is made up of an L-shaped mesh part 1 for the bottom 3 and one side of the beam, a mesh part 12 for the center of the beam, a mesh part 16 for the other sides of the beam, and a mesh part 25 for the upper part of the beam. These mesh parts 1, 12, 16 and 25 are manufactured in advance in a factory and then delivered to a construction site. The mesh part 1 is first set in place, the lower main iron bars 9 are welded to the mesh part 1, the mesh parts 8, 12 and 16 are orderly welded, the upper iron bars 21 are welded to the upper part of the beam, and the upper mesh part 25 is combined to complete the assembly of the beam. Thus, labor can be reduced and the assembling accuracy of the beam can be raised.