A core has first to third magnetic leg portions. First and second windings wound on the first and second magnetic leg portions, respectively, are connected in series to constitute a first reactor. A third winding wound on the third magnetic leg portion constitutes a second reactor. A magnetic field produced from the first reactor and a magnetic field produced from the second reactor reinforce each other in the second magnetic leg portion, but weaken each other in the first magnetic leg portion. In accordance with increase in currents, the operation of the first and second reactors changes from a magnetically uncoupled mode in which the first and second reactors operate in a magnetically non-interfering state to a magnetically coupled mode in which the first and second reactors operate in a magnetically interfering state.