We studied reactor size reduction by applying a slot jet cooling method. In power electronics products such as large power conversion equipments for industry applications (conversion equipments), reactors take up a large proportion of the volume and weight, and miniaturization of reactors is an important issue in reducing the size of conversion equipments. Reactors cooled by forced air convection are generally almost the same size as those cooled by natural air convection. We applied a slot jet cooling method to make cooling air flowing out from two or more slots impinge on the reactor surface at high speed. There have been many reports concerning jets from single slots or two or more slots distributed in parallel. However, many of the details of mutual interference and the exhaust flow along the direction of the slot after jet impingement on a surface remain unknown. Therefore, these basic findings were experimentally verified, and an optimum slot jet giving the greatest improvement in cooling performance, while taking account of exhaust flow, was found. By applying this slot jet cooling, the mass was reduced by about half compared with a conventional reactor.