PURPOSE: To make it possible to detect the positional slippage of each hole during drilling operation under through-hole of a printed board by detecting the positional slippage of a hole by means of electrical conduction between a right and a left patterns or an up and a down patterns on a printed board for detection of hole positional slippage.

CONSTITUTION: On a printed board, a right and a left patterns 3, 3' and an up and a down patterns 4, 4' are set with which internally touch the planned drill holes (not drilled yet) corresponding to the drilling points on a work, a printed board. These patterns 3, 3' and 4, 4' are drawn in one stroke with no break with a width W equal to the hole slippage allowance and in the sequence of drilling (as shown with the arrow A). If the drill should slip within the positional slippage allowance, either one of the pattern 3, 3' or 4, 4' or both of them will remain open, not be short-circuited, while the drill is descending. In this way, the positional slippage of a hole greater than the allowance can be detected for each hole.