Abstract: A main pipe is lined using a pipe lining material that blocks the opening of a lateral pipe branching from the main pipe. An in-pipe work robot is provided with a cutting nozzle for spraying a pressurized fluid material or pressurized granular material onto the pipe lining material of the main pipe. The cutting nozzle is moved to a predetermined position inside the main pipe and rotated about a vertical axis by a rotary mechanism to cut the lining material between the first and second pipes by the pressurized fluid material or pressurized granular material sprayed from the cutting nozzle in order to form an opening for communicating the main pipe with the lateral pipe. The cutting nozzle can be moved in desired positions to drill holes accurately without damaging the internal peripheral surface of the lateral pipe.