To provide an underground wall used for a main body wall of an underground structure capable of reducing a construction period of time in comparison with the conventional technology and, at the same time, realizing the effect of the curtailment of a construction cost.

Flange steel plates 16 are provided to both ends of a web steel plate 15 having a specific height (H) dimension, one steel member 10 with an approximately H-shaped cross section having female joints 17 on the ends of the flange steel plates 16 on one side or both sides and the other steel member 10 with an approximately H-shaped cross section having male joints 18 capable of being locked by forming the female joints 17 and a fitting space allowance 21 in the ends of the flange steel plates 16 are constituted, the female and male joints 17 and 18 are locked with each other to embed the steel members 10 in the ground 11 or an elapsed hardener injected into a ground excavated trench 13, and the fitting space allowance 21 locking the female and male joints 17 and 18 therewith is filled with filler such as cement milk 22 or the like to construct an underground wall 12 used for a main body wall of an underground structure.