PURPOSE: To obtain an alloy thin film having high saturation density, low coercive force and high permeability and exhibiting high permeability even in a high-frequency band by specifying the composition.

CONSTITUTION: This thin film consists of the microcrystalline phase consisting essentially of Fe of body-centered cubic structure and having ≤10nm average crystal grain diameter and an amorphous phase consisting essentially of the compd. of N (nitrogen) and M as at least one kind of element selected from a group of the rare-earth elements, Ti, Zr, Hf, V, Nb, Ta and W, and the amorphous phase occupies at least 50% of the structure. The composition expressed by Fe1MbNc is preferable. In the composition, the compositional ratio of (a), (b) and (c) is atomic%, 60≤a≤80, 10≤M≤15, and 5≤N≤30. A thin film with an amorphous phase as the center is formed on a substrate by vapor deposition such as high-frequency sputtering conducted while passing gaseous nitrogen to produce the alloy thin film.