A boring device is inserted into a lateral pipe to bore a pipe lining material inserted into a main pipe across the lateral pipe intersecting therewith and a portion of which blocks an opening of the lateral pipe that communicates with the main pipe. The boring device has a rotatable shaft, a bearing member for rotatably bearing the rotatable shaft, and a boring blade mounted on the rotatable shaft for rotation therewith to bore through the portion of the pipe lining material that is blocking the opening of the lateral pipe. A supporting member is mounted on one side of the bearing member. An inflatable bag is mounted on the other side of the external periphery of the bearing member. The inflatable bag is inflatable in the lateral pipe so that the inflatable bag and the supporting member are pressed against an internal peripheral surface of the lateral pipe.