To provide an impeder which shows an excellent durability when used under a high frequency and a large power by gluing cores together composed of a magnetic metal material in one or more forms of wires whose diameter is a prescribed value or smaller, foils whose thickness is a prescribed value or smaller, or powder whose particle size is a prescribed value or smaller, to form an assembly using an insulating adhesive whose durable temperature is a prescribed value or higher.

The impeder is provided with a core of a magnetic material and a protective cooling means of the core. The diameter of wires, the thickness of foils, and the particle size of powder are all 25 μm or smaller, and the durable temperature of the insulating adhesive is 150°C or higher. The calorific value of the core made of magnetic metallic material increases in proportion to the approximate square of the wire diameter, the thickness, or the particle size, however, it is equal to or smaller than the calorific value of a core made of a magnetic oxide when the dimension is 25 μm or smaller. A core made of metallic magnetic materials causes a self-heating due to a high-frequency eddy current, the insulating adhesive deteriorates and its adhesive power decreases, however, the core is not thermally destroyed as far as the wire diameter, the thickness, and the particle size are all 25 μm or smaller, and the durable temperature is 150°C or higher.