Conceptual design studies of a next generation tokamak fusion reactor, the Fusion Experimental Reactor (FER) at JAERI are described.Recent design efforts have been focused on elaborating two typical design concepts, one based on the best existing physics data bases and the other on rather conservative physics basis, to be capable of achieving a self-ignited, long burning DT plasma and demonstrating the feasibility of key fusion reactor technologies. Various flexibility scenarios to enhance the machine capabilities to realize higher reactor-core performance have also been developed and incorporated in the design in accordance with an overall plan of phased construction and operation program of FER.