A magnetic core for use in a saturable reactor made of an Fe-based soft-magnetic alloy comprising as essential alloying elements Fe, Cu and M, wherein M is at least one element selected from the group consisting of Nb, W, Ta, Zr, Hf, Ti and Mo, and having an alloy structure at least 50% in area ratio of which being fine crystalline particles having an average particle size of 100 nm or less. The magnetic core has control magnetizing properties of a residual operating magnetic flux density DELTABb of 0.12 T or less, a total control operating magnetic flux density DELTABr of 2.0 T or more, and a total control gain Gr of 0.10-0.20 T/(A/m) calculated by the equation: Gr=0.8x(DELTABr-DELTABb)/Hr, wherein Hr is a total control magnetizing force defined as a control magnetizing force corresponding to 0.8x(DELTABr-DELTABb)+DELTABb.