To provide a drilling guide device of a drilling machine to correctly cut into a pipe for drilling when a branch pipe connection hole to connect a branch pipe to a conduit used for sewage and water supply embedded underground is drilled.

A drilling device 7 comprising a drill 3, a supporting column 6, and a drill driving shaft 4 which is supported by the supporting column 6 and can be vertically moved is provided on an upper surface side of a base 2, and a holding device 8 of a pipe body P is constituted on the lower surface side of the base 2 by providing an upper tube receiving part 91 to hold the pipe body P and a tube side pressing body 11 having a tube side receiving body 10 and a regulating device 12 in the vicinity of a drill advancing hole 1 on the lower surface side of the base 2. The pipe body P is held by the holding device 8 in a point-contact manner from three direction of the pipe body toward the center of the pipe body P, the pipe body P is held so as to prevent uncontrollable movement, the drill 3 is cut from the drill advancing hole 11 of the base 2, and a truly circular branch pipe hole is cut in the pipe body P.