Abstract: A method of forming an underground cavern aiming at forming an underground cavern of a large scale at a very deep place in soft ground and soft rock layers safely and economically, and a tunneling machine suitable for use in carrying out the method. This method of forming an underground cavern consists of the steps of forming a ground reinforcing zone around a portion intended to be hollowed out before froming an underground cavern to be built, and then excavating the inside surrounded by the ground reinforcing zone. Further, the above-mentioned tunneling machine comprises a ring-shaped body having a first ring, a second ring, and a third ring; a boring device mounted between the second ring and propelling jacks for boring a plurality of holes extending radially from the inner surface of the tunnel into the ground; and a reinforcing material filling device mounted also between the second ring and the propelling jacks for filling a reinforcing material and a grout into each of the holes.