Prior to lining a main pipe, a marker configured as a receiving coil 33 is disposed at the center 12b of a lateral pipe opening or at a position separated a predetermined distance S from the center thereof. An in-pipe robot 20 is loaded coaxially or separated by the predetermined distance with a rotatable drilling blade 31 for drilling a hole in a main pipe lining material 13 that blocks the lateral pipe opening and a transmitting coil 40 that detects a marker position. After lining the main pipe, the drilling blade and the transmitting coil are moved in conjunction in the main pipe longitudinal direction and in the main pipe circumferential direction to drill a hole in the main pipe lining material at a position where the electromagnetic coupling of both the coils becomes maximum. The center of rotation of the drilling blade coincides with the center of the lateral pipe opening at the position where the electromagnetic coupling of both the coils becomes maximum, so that a hole can be drilled precisely in the main pipe lining material in alignment with the lateral pipe opening.