To improve productivity and reduce the cost by providing scale divisions near a coil on a plate-like member to facilitate accurate adjustment of the inductance.

A coil 3 is mounted on a plate-like member 1 with its mounting part inserted in holes from the upper surface of the member and soldered to conductors, thus connecting the coil 3 to constitute a printed board P. When the coil 3 is mounted on the member 1, the lower part of a coil winding 3a contacts a top face 1c of the member 1. Scale divisions 4 are marked near the side and end of the coil winding 3a of the air-core coil 3 on the surface 1c of the member 1 by printing a resist, etc., thus serving as a mark for adjusting the spacings of the windings at the time of the inductance adjustment.