PURPOSE: To promote work efficiency by extending frame fixed jacks to fix a horizontal frame to both side walls after an inclination of the horizontal frame is detected by an inclination detector, and excavating by means of excavators mounted on a horizontal movement truck.

CONSTITUTION: The horizontal frame 2 mounting an inclination detector 3 and a plurality of frame fixed jacks 4 is provided to a trench bottom treatment device 1 for an underground continuous wall building method. The horizontal movement truck 5 is borne on the horizontal frame 2 lengthwise of an underground trench so that it is capable of horizontal movement, and an excavating frame 7 is placed under the movement truck 5. The trench bottom treatment device 1 is hung down to the trench bottom side, and after an inclination of the horizontal frame 2 is detected by the inclination detector 3, it is fixed horizontally. Then, the truck 5 is moved horizontally, and excavators 10 and 11 mounted on the truck 5 excavate trench bottom.