Provided is an iron nitride-based magnetic powder that comprises magnetic particles having a mean particle size of at most 20 nm. The magnetic particle has a core of a main phase of Fe 16 N 2 and has, on the outer side of the core, an oxide phase derived from a metal Fe phase formed by reduction of iron nitride. In relation to the weatherability index Δσs and the saturation magnetization σs thereof, the magnetic powder satisfies Δσs ≤ 0.8 x σs - 30. In this, Δσs = (σs - σs 1 )/σs x 100. σs 1 means the saturation magnetization of the magnetic powder kept in an atmosphere of 60°C and 90 % RH for 1 week. The powder can be obtained by exposing powder particles having a main phase of Fe 16 N 2 to a reducing gas to partly reduce the region of the surface of the particles into a metal Fe phase (gradual reduction) followed by exposing them to an oxidizing gas to oxidize a part of the surface of the metal Fe phase into an oxide phase (gradual oxidation).