PURPOSE: To construct a main exhaust duct underground in an atomic power plant in a short time by a method in which a reinforced concrete unit having a duct in its inside is hung down into an excavated trench, while the ducts of each unit are connected with each other, and then the trench is backfilled with soil.

CONSTITUTION: The necessary portion of the ground is excavated to form a trapezoidal trench 13, and mat concrete 14 is placed on the bottom. Then, reinforced concrete units 10 having main exhaust ducts in its internal spaces are hung down into the trench 13 by a lifter 16 to place them on the mat concrete 14 in such a way as to expose its upside alone and then set on a rail 15 buried in the mat concrete 14. The fixing metal 11 at the bottom of the unit 10 is welded to the rail 15 for fixing the unit 10. In this way, the units 10 are orderly connected, and the ducts inside the units are connected with each other by welding. The units are laid over the whole length of the trench 13 and lined with a water-proofing material, and the trench is backfilled with soil.