PURPOSE: To safely and easily construct an underground structure by a method in which the inside area of a steel framework consisting of V-shaped steel plates and protective steel plates is excavated, excavated soil is discharged, and holding bases are arranged beneath the steel framework and fixed to the steel framework.

CONSTITUTION: A steel framework consists of V-shaped steel plates 3 and 3', a protective steel plate 8, reinforcing steel plate 5, a projected beam form 20, and an anchoring iron bar 11. The steel plate 3 is slightly projected to the peripheral side from the protective steel plate 8, and a cavity 14 and a hanging metal 15 are provided to a holder base 4. Concrete is placed in the steel framework, the inside area of the steel framework is excavated, the excavated soil is discharged, and the steel framework is settled. The holder bases 4 are arranged at invervals beneath the steel framework, the hanging metal 15 is welded to the reinforcing steel plate 5, and the holder bases 4 are fixed to the steel framework. Concrete is placed into the space formed by the holder bases 4, the steel plate 3', and broken stones to form a slab 3.