As performance and speed of heavy duty vehicles become higher there are much market needs of higher performance of brake system for higher safety. Recently load on wheel brake mainly for high speed vehicles is increasing by lowered engine brake force and exhaust brake force caused by adoption of small swept volume diesel engine with turbo and inter cooler coupled with fast ratio final gear for fuel economy of heavy duty vehicles. Besides that, load on wheel brake is also increased by reduced rolling resistance of tire and aerodynamic drag force. In the past wheel brake size was increased (mainly width of brake drum was increased) to remedy the situation, but now because of space problem further increase of width is impossible. As a result various kinds of study were conducted such as material change of lining, improvement of cooling performance of brake drum, adoption of retarder and etc, from among them retarder was the best choice. But there were many problems of electro-magnetic or hydrodynamic retarders also such as heavy weight, space for installation and etc, and retarder is not so popular in Japan. Under these circumstances, authors succeeded for the first time in the world to develop epoch-making eddy-current retarder featuring light weight, compact and easy installation by using very powerful Nd-Fe-B base permanent magnet. This paper describes about this new retarder and improvement of safety at the time of brake application while running at high speed based on test results of express way on mountainous district. This new retarder can be incorporated to conventional brake system as an integral part, and for improvement of brake safety retarder is to be used mainly at high speed while conventional wheel brake system is to be used at low speed running or in an emergency. This paper is to deal with new concept of braking system of heavy duty vehicles in this way.