A reactor in which two first cores 1 made of lamination and I-shaped and two second cores 2 made of lamination and I-shaped are disposed such that the two first cores are parallel to each other, the two second cores are parallel to each other and that the two second cores having respective coils 4 therearound are perpendicular to the two first cores, thereby forming a rectagular shape, wherein there are provided a first spacer 10 and a second spacer 20, which have on their respective outward side faces two protrusions 13, 23 for positioning each first core and two other protrusions 14, 15 for holding longer sides of each first core with one thereof having at its tip end an engaging mechanism to secure each first core, and which have on their respective inward side faces four protrusions 17, 27 for positioning two neighboring corners of the respective second cores and an elevated portions 16, 26 provided between the four protrusions and elevated in a direction perpendicular to the length of the first cores. As a result, the reactor has a good workability and a reduced number of parts.