A reactor using a composite magnetic core in which a ferrite core and a soft magnetic metal core are combined. The reactor is composed of a pair of yoke portion magnetic portions composed of a ferrite core, winding portion core(s) disposed between the opposite planes of the yoke portion cores, and coil(s) wound around the winding portion core(s). The winding portion core(s) is/are made of a soft magnetic metal core, and the cross sectional area of the part for winding the coil of the winding portion core is substantially constant. When the cross sectional area of the part for winding the coil of the winding portion core is set as S1, and the area of the parts opposite to the yoke portion cores in the winding portion core(s) is set as S2, the area ratio S2/S1 is set to be 1.3 to 4.0.