PURPOSE: To efficiently increase a size by means of excavating water, by a method wherein a slewing rod 7c, provided at its forward end with an excavating bit and a soil discharge port, is supported to the lower end of the stationary rod of an excavating device in a manner that it can be freely slewed, and an excavating nozzle is attached to the slewing rod so that it is movable in parallel.

CONSTITUTION: A stationary rod 7b of an excavating device and slewing rod 7a, supported to the lower end of the stationary rod so that it can be freely slewed,are held on a straight line for rotation, and excavation is effected down to a given depth by means of an excavating bit 17. An excavated surface 31a is excavated through injection of excavating water 29 through a nozzle 16 attached to the slewing rod 7c, and the slewing rod 7c is slewed through a driving cylinder 9 in response to an increase in size to approach the nozzle 16 to the excavated surface 31a. In which case, the nozzle 16 is always horizontally held by means of a parallel moving mechanism 15. This causes a distance between the nozzle 16 and the excavated surface 31a to be held at an optimum value, and enables efficient increase of the size of an excavated hole.