PURPOSE: To improve the rectilinearity of an underground wall, uniformalize the mixed state in the horizontal direction of an underground continuous wall formed from the mixture of the soil and sand at the original ground and the a solidifying liquid, shorten the execution period and carry out the excavation in the horizontal direction by preventing a horizontal direction excavating means which is vertically long, from being tilted by the soil pressure.

CONSTITUTION: An underground continuous wall forming device is equipped with an on-ground shifting device 1 which shifts on the ground, horizontal direction excavating device 2 which is vertically long and inserted into the ground, and supported on the on-ground shifting device for the excavation in the horizontal direction and a means for applying the force in the horizontal direction on the upper and lower parts of the horizontal direction excavating device 2. Accordingly, even if the soil pressure resistance is applied, the tilt of the horizontal excavating device 2 which is vertically long is prevented, and an underground continuous wall can be formed by continuously excavating the underground part in the horizontal direction.