PURPOSE: To reduce the thickness and to enhance flat rigidity by altering the sizes of coil poles on the front and rear surfaces of a board and providing reinforcing pattern conductors on the side of small size coil pole across a plurality of large size coil poles opposing to the small size coil pole.

CONSTITUTION: The areas of coil poles of front and rear surfaces of an insulating sheet 3 are altered while holding symmetry to the center line of the poles, the pattern conductor 1 of the pole on the front surface is formed in the same phase as the pattern conductor 1 of the poles on the rear surface side, and connected in series via a through hole electrode 5 in the pole. An annular reinforcing pattern semiconductor 7 is provided at the inner peripheral side of the first coil pole 1'. The conductor 7 is so arranged as to be superposed on a plurality of second coil poles 2', and the annular reinforcing pattern conductor 7 is provided to reinforce the rigidity of the sheet between the poles.