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 ferrite, winding portion core(s) disposed between the opposite planes of the yoke portion cores, and coil(s) winding around the winding portion core(s). The winding portion core(s) is/are formed using a soft magnetic metal core with a substantially constant cross sectional area. Junction portion cores composed of soft magnetic metal powder cores with a tabular shape are disposed at the spaces where the winding portion core(s) face(s) the yoke portion cores, and the area of the part where the junction portion core faces the yoke portion core is made to be 1.3 to 4.0 times that of the section of the winding portion core.