PURPOSE: To substantially reduce a time required for drilling reference holes in a base plate, by actually detecting only three reference mark positions and obtaining the rest of the reference mark positions through calculation.

CONSTITUTION: Many reference marks 32 are provided in a lattice pattern in a base plate 30, but detection of coordinates and drilling are performed for all the reference marks 32 about the first sheet of the base plates 30 of the same kind. The detected coordinates are all stored in a memory in a control unit. Next, the coordinates are actually detected only about the three reference marks 32 corresponding to those A, B, C for the second sheet of the base plate 30. Accordingly, the necessity for detecting work is eliminated for those except the actually detected reference mark 32.