PURPOSE: To reduce heat generated by friction between each printed wiring board and a drill bit so as to prevent smearing the inside wall of a via hole by retaining water-sealing microcapsules in each sheet made of a water soluble skid.

CONSTITUTION: To make a hole in printed wiring boards 4, a floor plate 6, a boring assisting sheet 2, one of the printed wiring boards 4, a boring assisting sheet 2, the other printed wiring board 4 and a boring assisting sheet are transferred in that order from the lowermost layer. The boring assisting sheets 2 each comprise a sheet made of a water soluble skid and retaining microcapsules 1 in each of which water is sealed. When the sheets are drilled using a drill bit 7, the microcapsules 1 are broken and the water sealed therein is dissipated, cooling the drill bit 7. The water dissolves the water soluble skid which is the main component of the assisting sheets 2, thus reducing friction between the drill bit 7 and each of the printed wiring boards 4.