PURPOSE: To increase coupling coefficient and frequency bandwidth, and reduce insertion loss, by a method wherein a magnetic layer sandwiched by insulating layers and a conductor layer constituting two or more coils which reciprocate on the upper side and the lower side of the insulating layers and go round the magnetic layer are provided, and the upper surface of the insulating layer in contact with the lower surface of the magnetic layer is made flat, which magnetic layer is formed in a flat plate type.

CONSTITUTION: A closed magnetic path, which has an aperture at the center of the surface of a substrate 1 composed of insulating material and circulates in a plane parallel to the substrate surface, is constituted. A first insulating layer 3 and a second insulating layer 5 sandwich the lower surface and the upper surface of a magnetic layer 4. A conducting layer is wound so as to alternately pass the upper side and the lower side of the insulating layers from one end of the magnetic layer 4 to the other end thereof through the central aperture part. That is, a first conducting layer 2 and a second conducting layer 6 are arranged, and the conducting layer has at least two coils. In particular, the upper surface of the first insulating layer 3 is flattened, and the magnetic layer 4 thereon is formed in a plane type. Thereby, the magnetic layer 4 is not bent, so that the leakage of magnetic flux is reduced and coupling coefficient is increased. Thus a thin film transistor of wide frequency band can be obtained.