An object of the present invention is to provide a small-sized and highly heat-dissipative reactor and a DC-DC converter using the reactor. A reactor according to the present invention includes a plate bus bar, a core, and a heat sink. The core includes a middle leg portion. The heat sink cools the plate bus bar. The plate bus bar is formed such that a winding axis of a winding including the plate bus bar passes through the middle leg portion. A main surface of the plate bus bar is disposed in parallel with a direction of the winding axis and thermally connected to the heat sink via an insulating layer.