NOVELTY - The reactor (2) has two coils (12a, 12b) located in a flat case (10). The coils are wound in opposite direction from each other and arranged adjacent to each other in radial direction of the coils. Core material covers the coils in the flat case. The coils are aligned in direction intersecting axial direction (CL) of winding of the coils. The core material includes high-permeability core material covering first portions of each of the coils, and low-permeability core material (4) covering second portions of each of the coils not covered by the high-permeability core material.    USE - Reactor for being arranged between stacked coolers of a power converter that is utilized for converting output power of a battery for driving a traveling motor such as induction motor and PM motor, in an electric vehicle i.e. hybrid electric vehicle.    ADVANTAGE - The reactor ensures that outer peripheries of the coils in radial direction are contacted with each other, so that the flat case is downsized since the adjacent portions of the coils contact each other and the coils are arranged in the flat case in a tightly fitted manner, thus downsizing the reactor. The reactor generates magnetic fluxes opposing each other in the coils when current flows through the coils, so that the opposing magnetic fluxes are joined to each other to generate large magnetic flux so as to increase inductance of the reactor, and make the reactor to be cooled easily. The reactor suppresses leakage of the magnetic flux in adjacent portions of the coils since the large magnetic flux generated in the coils passes through the high-permeability core material with high permeability so as to suppress generation of eddy current around the reactor caused by leakage of the magnetic flux, thus suppressing eddy current loss. The low-permeability core material is stored around the high-permeability core material, so that the magnetic flux passes through the low-permeability core material even when the high-permeability core material is magnetically saturated so as to prevent magnetic saturation of the core materials in the flat case. The reactor ensures that a stacked unit increases adhesion between a semiconductor module and the cooler and adhesion between the reactor and the cooler when pressurized in stacked direction, thus improving heat transfer efficiency to the cooler.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of a reactor.    Axial direction (CL)    Reactor (2)    Low-permeability core material (4)    Flat case (10)    Coils (12a, 12b)