To construct quality uniform ditch walls in a way that a ditch is excavated, and excavated earth and a solidifying liquid agent are mixed together within the ditch and used for constructing the ditch walls.

Digging wheels 12, 12' are operated in earth 3 to dig it and form a ditch 1 there. A solidifying liquid agent is supplied from a liquid line 68 while digging the earth 3, mixed with dug earth to solidify it to form the trench wall. The solidified earth is used for constructing a ditch wall. While digging the earth 3, gas is also supplied in the ditch 1 through a gas line 58, besides the solidifying liquid agent, to agitate the suspension of earth and solidifying liquid agent to make the ditch wall uniform.