PURPOSE: To provide an automatic control means for the depth of the counterbore of an identification mark provided on each layer used for position identification in boring a reference hole through a photomultilayer printed board.

CONSTITUTION: Chips of a printed board 13 are allowed to pass through a suction tube 7 in such a manner that chips from a copper foil portion and chips from an insulating portion are alternately fed. A detection coil 8 generates a high frequency magnetic field, detecting passage of metals. Alternatively, chips of copper foil are detected by a semiconductor laser and a light receiving element both of which are provided to a suction opening 6. The number of times of detection of the copper foil by the detection coil 8 (i.e., the number of layers) is counted, whereby control of the depth of a counter bore, which has conventionally been done by hands, can be automated.