A drilling apparatus has a work robot mounting a drilling blade and moving inside a main pipe and a photodetector for detecting the contour of an opening image formed on the inner surface of a pipe lining material by illumination light. The opening image is scanned, and a two-dimensional image corresponding to the opening image is generated based on the contour points P0 to P8 detected by the photodetector for display on a display unit. The amount of positional deviation between the center of the two-dimensional image on the display unit and the axial center C of the rotary shaft of the drilling blade is calculated, and the pipe lining material is drilled by moving the drilling blade by the amount of positional deviation in a direction in which the positional deviation is eliminated.