A printed wiring board includes three or more than three through holes. An inner wall of the through hole is covered by conductive coating. Same size leads of an electronic component are inserted into the through holes. The through holes are soldered by dip soldering the printed wiring board in melting solder. The through holes have two or more diameters. The diameter of the through hole having more adjacent through holes is not larger than the diameter of the through hole having less adjacent through holes.