In this paper, we propose a level-set based topology optimization method for designing a reactor, which is used as a part of the DC-DC converter in electric and hybrid vehicles. Since it realizes a high-power driving motor and its performance relies on its component, i.e., reactor core, it is valuable to estab- lish a reasonable design method for the reactor core. Boundary tracking type level-set topology optimization is suitable for this purpose, because the shape and topology of the target structure is clearly represented by the zero boundary of the level-set function, and the state variables are accurately computed using the zero boundary tracking mesh. We formulate the design problem on the basis of electromagnetics, and derive the design sensitiv- ities. The derived sensitivities are linked with boundary track- ing type level-set topology optimization, and as a result, a useful structural optimization method for the reactor core design prob- lem is developed.