PURPOSE: To form an arbitrary circular under-hole with a simple device in the stage of cutting and forming a circular under-hole on the surface or the like of a pipe by rotating a rollered top mounted with a cutting torch one turn horizontally on the cutting plane.

CONSTITUTION: In the stage of cutting a circular under-hole 12 on the surface of a pipe 11 placed on a supporting table 13, a revolution transmitting device 23 which is rotated freely by a motor 22 is mounted to the projecting end of a stanchion 21, and the preceding end of a revolving fulcrum shaft 25 is set at the center of the hole 12. A holder 30 is mounted to the outer arm part 24 of the shaft 25, and an oscillating device 100 consisting of guiding rollers 34 and a top 33 provided with a cutting torch 36 is mounted via a fulcrum pin 32 to the holder 30. While the device 100 is rotated with the shaft 25 as a rotating axis, the holder 30 is pressed by means of an air cylinder 20 to a pipe 11 and the hole 12 is cut and formed with the torch 36.