Provided are a reactor which can perform step-up/step-down operations and soft switching with a small size and a smaller leakage inductance, and a method of adjusting a leakage inductance of the reactor. A reactor 1A includes a magnetic core 10A having a pair of inner core portions and forming a closed magnetic path, a main coil 11A having main coil elements 11a and 11b, and a sub-coil 12A having sub-coil elements 12a and 12b. The coil elements 11a and 12a are concentrically layered over one of the inner coil portions, and the coil elements 11b and 12b are concentrically layered over the other inner coil portion. One end portion of a wire 11w of the main coil 11A and one end portion of a wire 12w of the sub-coil 12A are joined to each other. A spacing between adjacent turns constituting the sub-coil element 12a (12b) is wider than that between adjacent turns constituting the main coil element 11a (11b). Thus, the reactor 1A has a smaller leakage inductance.