To provide a reactor whose number of components is small and that exhibits excellent assemblability. A reactor 1 includes a coil 2 having a pair of coil elements 2a and 2b, and a core unit 3 having a pair of intermediate core portions 30 around which the coil elements 2a and 2b are respectively disposed. The core unit 3 is formed to be annular by a combination of intermediate core pieces 31 that structure intermediate core portions 30, a pair of end core pieces 32 that clamps a pair of intermediate core portions 30 disposed in parallel to each other, and gap members 3g each disposed between each ones of the core pieces. The end core pieces 32 respectively include clamping faces 321 for clamping the intermediate core portions 30, each structured with a single plane. An installed face 320l of each of the end core pieces 32 projects further than the installed face 31l of the intermediate core pieces 31. One gap member 3g is interposed between the end core piece 32 and the intermediate core portion 30, and relative permeability of the gap member 3g is greater than 1. This structure simplifies the shape of the core pieces, and allows the number of components to be small despite the core unit 3 partially projects.