PURPOSE: To reduce the amount of soil excavation as well as the concrete volume required for a side wall and a bottom slab, and in addition, eliminate the need to remove an outer guide wall which is built outside a continuous underground wall, by connecting the top of the continuous underground wall to the external guide wall with dowel joints.

CONSTITUTION: Since the top of a continuous underground wall 2 is connected with dowel joints 7 to an external guide wall 6 built outside the underground wall 2, the weight of the guide wall 6 can be added to the weight of the structure of an underground tank. Therefore, safety of the structure of the underground tank against the buoyancy of underground water can be ensured and a side wall 4 and the bottom slab 3 can be built in thickness having strength required for bearing the pressure of surrounding soil, resulting in a reduction in thickness of the side wall 4 and the bottom slab 3 and in the amount of soil excavation. As the outer guide wall 6 is used as part of the weight of the structure of the underground tank by connecting it to the continuous underground wall 2, the removal work of the outer guide wall 6 becomes unnecessary.