To reduce eddy current loss of a printed wiring board by forming parallel linear holes through an insulating substrate, and providing a printed coil forming a spiral conductor pattern having an equal width around the part of the insulating substrate between the linear holes, including both surface of the part and internal surfaces of the holes to the substrate together with other circuits.

Parallel linear holes 2 are formed through an insulating substrate 1, and a conductor pattern 3 having an equal width is formed spirally around the part of the substrate 1 between the holes 2, including both surfaces of the part and the internal surfaces of the holes 2. The holes 2 are composed of two or more narrow and long through-holes formed in parallel with each other, and an axis for forming the conductor pattern 3 of a printed coil 4 is formed in a printed wiring board. The conductor pattern 3 can be formed by plating both surfaces of the part between the holes 2 and the internal surfaces of the holes 2 after unwanted parts on the surfaces are coated with a resist, by eliminating the unwanted parts of a conductive film with a laser beam after the film is formed on both surfaces of the part and the internal surfaces of the holes 2, and so on.