Abstract: An underground excavator includes a chamber defined in the forward section of an excavator body for receiving excavated material such as soil, sand, gravel, clay and their mixtures. A rotary cutter is mounted to the forward end of a rotary shaft for penetrating through the chamber for axial rotation and simultaneous rocking in an axial direction. The cutter has a conical cutter face plate having holes for passing the excavated material of a diameter smaller than an opening disposed in the chamber. Any gravel or the like present in the advancing route to the underground excavator and smaller than the holes of the cutter is received into the chamber to be discharged from the excavator. Any gravel larger than the holes or the like not receivable into the chamber is moved quickly along the conical cutter face plate toward peripheral ground layer by being subjected to the rocking motion of the conical cutter face plate simultaneously with the rotation of the cutter during advancement of the excavator.