PURPOSE: To excavate even highly cohesive soil in wider width by a method in which in a hydraulic jet nozzle consisting of a straightforward nozzle through which water jet is directed practically straightforwards and a fan-shaped nozzle through which water jet is expanded in a fan shape, both the straightforward nozzle and the fan-shaped nozzle are set adjacently in the widthwise direction of a trench.

CONSTITUTION: A fan-shaped nozzle 6 is of a shape in which the tip of a circular pipe 7 is gradually, crushed, and its jet port 8 is of a flat and oblong shape of a higher diameter than that of the pipe 7. High-pressure water is jetted from the nozzle 6, and compressed air of a higher pressure than water is jetted from the peripheral air path 9 to make up an air-mixing jet nozzle. The nozzles 6 are provided at intervals of effective range of jets (c) at the center of a casing 1, high-pressure straightforward air-mixing nozzles 10a are set on both sides of the nozzle 6, and low-pressure straightforward air-mixing nozzles 10b are also set as needed.