A soft magnetic alloy thin film includes a fine crystalline phase and an amorphous phase. The fine crystalline phase includes an average crystalline grain size of 10 nm or less in diameter and has body-centered cubic structure mainly composed of Fe. The amorphous phase has a nitrogen (N) compound as the main composition and occupies at least 50% of the structure of the thin film. An element M is incorporated at least in the amorphous phase, and includes at least one element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, W, and rare earth metal elements. A plane-type magnetic device is made using this thin film.