PURPOSE: To remove a foreign substrate such as resin or the like plugging up a through-hole assuredly at a high speed by a method wherein laser rays, which are made to converge so as to be slightly smaller than the through-hole in diameter, are directed toward the inside of the through-hole of a printed wiring board.

CONSTITUTION: A printed wiring structure 1 provided with origin holes 5a and 5b, which is not subjected to an outer-shape-cutting process yet, is placed on a table 4 on the basis of the origin holes 5a and 5b. Next, laser rays 8 whose diameter is smaller than that of the through-hole 3 are directed to the inside of the through-hole 3 as being controlled by a memory medium which stores a converted data concerning the size and the position coordinates of the land provided to an outer pattern of the printed wiring structure 1. By these processes, a forein substance inside a through-hole can be assuredly removed at a high speed.