A transformer wherein the upper portions of cores are supported by a first supporting member disposed on first end surfaces of the upper portions of the cores, and a second supporting member disposed on second end surfaces of the upper portions of the cores, the first and second supporting members extend in the direction perpendicular to the faces of a magnetic material, and the cores are interposed between the first upper core supporting member and the second upper core supporting member; the first and second upper core supporting members are provided with hooks, the hooks of the first supporting member extending toward the second supporting member and the hooks of the second supporting member extending toward the first supporting member; bridging members are disposed on the opposing pairs of the hooks of the first and second upper core supporting members; and the cores are supported by the bridging members.