To attain accurate curve control by moving a drilling tool at constant angular velocity at the time of linear boring and periodically changing angular velocity at the time of curve boring.

A ground pit 1 is prepared into soil 2 by an elastic boring rod 3 consisting of a single pipe. A boring head 4 with an inclined plane 5 working as a drilling tool is fixed at the end section of the boring rod 3. When linear boring is conducted by the device, the drilling tool is moved substantially at constant angular velocity. When curve boring is performed, the angular velocity of the drilling tool is altered periodically. Accordingly, the gentle change of the direction, by which the boring rod 3 and the boring head 4 are not damaged at the time of drawing-in, and the uniform degree of curvature can be imparted within a curve range.