An Fe-based nanocrystalline alloy powder comprising a chemical composition, excluding inevitable impurities, represented by a composition formula of Fe a Si b B c P d Cu e M f, wherein the M in the composition formula is at least one element selected from the group consisting of Nb, Mo, Zr, Ta, W, Hf, Ti, V, Cr, Mn, C, Al, S, O, and N, 70 at % ≤ a ≤ 84.5 at%, 0 at % ≤ b < 6 at %, 0 at % < c ≤ 10 at %, 4 at % < d ≤ 11 at %, 0.2 at % ≤ e ≤ 0.53 at %, 0 at % ≤ f ≤ 4 at %, a + b + c + d + e + f = 100 at %, a particle size is 1 mm or less, a median of circularity of particles constituting the soft magnetic powder is 0.4 or more and 1.0 or less, the median of circularity being obtained by using the following equation as described in the description, φ = 4 π A / P 2 where φ: the circularity, A: the projected area of the particle in m 2, P: the perimeter of each particle of the particle in m, a degree of crystallinity of the Fe-based nanocrystalline alloy powder is more than 10 % by volume, the degree of crystallinity being obtained by analyzing the measurement results of X-ray diffraction with the whole-powder-pattern decomposition method as described in the description, an Fe crystallite diameter of the Fe-based nanocrystalline alloy powder is 50 nm or less, the Fe crystallite diameter being measured by XRD as described in the description, and an equivalent number n in the Rosin-Rammler equation represented by the following equation is 0.3 or more and 30 or less, the equivalent number n being measured by using laser particle size distribution meter as described in the description, R = 100 exp − d / c n where d: particle size in m R: volume ratio of particles having a particle size of d or more, in % c: a particle size in m when R = 36.8 % n: equivalent number. the M in the composition formula is at least one element selected from the group consisting of Nb, Mo, Zr, Ta, W, Hf, Ti, V, Cr, Mn, C, Al, S, O, and N, 70 at % ≤ a ≤ 84.5 at%, 0 at % ≤ b < 6 at %, 0 at % < c ≤ 10 at %, 4 at % < d ≤ 11 at %, 0.2 at % ≤ e ≤ 0.53 at %, 0 at % ≤ f ≤ 4 at %, a + b + c + d + e + f = 100 at %, a particle size is 1 mm or less, a median of circularity of particles constituting the soft magnetic powder is 0.4 or more and 1.0 or less, the median of circularity being obtained by using the following equation as described in the description, φ = 4 π A / P 2 where φ: the circularity, A: the projected area of the particle in m 2, P: the perimeter of each particle of the particle in m, a degree of crystallinity of the Fe-based nanocrystalline alloy powder is more than 10 % by volume, the degree of crystallinity being obtained by analyzing the measurement results of X-ray diffraction with the whole-powder-pattern decomposition method as described in the description, an Fe crystallite diameter of the Fe-based nanocrystalline alloy powder is 50 nm or less, the Fe crystallite diameter being measured by XRD as described in the description, and an equivalent number n in the Rosin-Rammler equation represented by the following equation is 0.3 or more and 30 or less, the equivalent number n being measured by using laser particle size distribution meter as described in the description, R = 100 exp − d / c n where d: particle size in m R: volume ratio of particles having a particle size of d or more, in % c: a particle size in m when R = 36.8 % n: equivalent number. φ: the circularity, A: the projected area of the particle in m 2, P: the perimeter of each particle of the particle in m, a degree of crystallinity of the Fe-based nanocrystalline alloy powder is more than 10 % by volume, the degree of crystallinity being obtained by analyzing the measurement results of X-ray diffraction with the whole-powder-pattern decomposition method as described in the description, an Fe crystallite diameter of the Fe-based nanocrystalline alloy powder is 50 nm or less, the Fe crystallite diameter being measured by XRD as described in the description, and an equivalent number n in the Rosin-Rammler equation represented by the following equation is 0.3 or more and 30 or less, the equivalent number n being measured by using laser particle size distribution meter as described in the description, R = 100 exp − d / c n where d: particle size in m R: volume ratio of particles having a particle size of d or more, in % c: a particle size in m when R = 36.8 % n: equivalent number. d: particle size in m R: volume ratio of particles having a particle size of d or more, in % c: a particle size in m when R = 36.8 % n: equivalent number.