A reactor 100 comprises a core 10 and a coil 20 embedded in the core 10. In the core 10 are provided gaps 30a, 30b extending from a surface of the coil 20 in a direction blocking a magnetic path. The gaps 30a, 30b have uneven widths, and include, at a position different from the surface of the coil 20, narrowest portions 31a, 31b having a smallest width.