PURPOSE: To efficiently, accurately and automatically detect an accurate drilling position without depending on human power in the case where a fastening bolt hole piercing a flange is drilled as a cast-out part of a necessary product information cast out so as to protrude from the flange face of a cast iron pipe is avoided.

CONSTITUTION: A drilling position detecting device has a oscillatable rotating roller 2 for bringing the surfaces of cast-out parts 3, where a letter, a letter row and a stamp seat on the flange face 4 of a rotating cast iron pipe 1 are mutually adjacent, mutually into pressure contact in common; and an eddy current displacement sensor for replacing oscillation being clue to the existence of the cast-out part into a voltage waveform, and thereby detects the longest range where the waveform having no cast-out part continues, and outputs it to the controller of a drilling device so as to fit the position of a fastening bolt hole on the basis of the above detection.