A high-reliability iron nitride-based magnetic powder with markedly improved weatherability with respect to deterioration over time of the magnetic properties in fine particles smaller than 25 nm is formed by adhering one or more of the elements Si and P to the surface of an iron nitride-based magnetic powder constituted primarily of Fe16N2 with an average grain size of 25 nm or less, where the total content of Si and P in the magnetic powder may be 0.1% or greater as an atomic ratio with respect to Fe. In particular, the invention provides an iron nitride-based magnetic powder such that the value ΔHc as defined by Equation (1) below is 5% or less and the value Δσs as defined by Equation (2) below is 20% or less. ΔHc=(Hc0−Hc1)/Hc0100  (1) Δσs=(σs0−σs1)/σs0100  (2) Here, Hc1 and σs1 are the coercivity and saturation magnetization, respectively, of the magnetic powder after being kept for one week at a constant temperature and constant humidity of 60° C. and 90% RH. Hc0 and σs0 are the coercivity and saturation magnetization of the magnetic powder before being kept at constant temperature and constant humidity.